Incidentally, coherence therapy (which I know is one of the things Chris is drawing from) makes the distinction between three types of depression, some of them being strategies and some not. Also I recall Unlocking the Emotional Brain mentioning a fourth type which is purely biochemical.

From Coherence Therapy: Practice Manual & Training Guide:

Underlying emotional truth of depression: Three types

A. Depression that directly carries out an unconscious purpose/function
B. Depression that is a by-product of how an unconscious purpose is carried out
C. Depression expressing unconscious despair/grief/hopelessness

A. Depression that carries out an unconscious purpose

Client: Mother who is still in pained, debilitating depression 8 years after her 5-year-old son died after being hit by a car. (To view entire session see video 1096T, Stuck in Depression.) The following excerpt shows the creation of discovery experiences that reveal the powerful purpose of staying in depression (a purpose often encountered with clients in the bereavement process).

Th: I want you to look and see if there’s some other side of you, some area in your feelings where you feel you don’t deserve to be happy again.
Cl: Probably the guilt.
Th: The guilt. So what are the words of the guilt?
Cl: That I wasn’t outside when he was hit (to prevent it).
Th: I should have been outside.
Cl: I should have been outside.
Th: It’s my fault.
Cl: It’s my fault.

(About two minutes later:)

Th: Would you try to talk to me from the part of you that feels the guilt. Just from that side. I know there are these other sides. But from the place in you where you feel guilty, where you feel it was your fault that your dear little boy got hit by a truck, from that place, what’s the emotional truth for you — from that place — about whether it’s OK to feel happy again?
Cl: ...I don’t allow myself to be happy.
Th: [Very softly:] How come? How come?
Cl: How come?
Th: Because if you were happy—would you complete that sentence? “I don’t allow myself to be happy because if I were happy—”
Cl: I would have to forgive myself. [Pause.] And I’ve been unwilling to do that.
Th: Good. So keep going. “I’m unwilling to forgive myself because—”
Cl: You know there are parts of me that I think it’s about not wanting to go on myself without him.
And if I keep this going then I don’t have to do that.
Th: I see. So would you see him again? Picture Billy? And just try saying that to Billy. Try saying to him, ”I’m afraid that if I forgive myself I’ll lose connection with you and I’ll go on without you.”
Cl: [With much feeling:] Billy, even though I can picture you as a little angel I’m afraid to forgive myself—that you’ll go away and I don’t want you to go away.
Th: Yeah. And see if it’s true to say to him, “It’s so important for me to stay connected to you that I’m willing to not forgive myself forever. I’d rather be feeling guilty and not forgiving myself than lose contact with you and move on without you.” Try saying that. See if that feels true.
Cl: [Sighs. With much feeling:] Billy, I just feel like I would do anything to keep this connection with you including staying miserable and not forgiving myself for the rest of my life. And you know that’s true. [Her purpose for staying in depression is now explicit and directly experienced.]

B. Depression that is a by-product of how an unconscious purpose is carried out

Client: Lethargic woman, 33, says, “I’ve been feeling depressed and lousy for years… I have a black cloud around me all the time.” She describes herself as having absolutely no interests and as caring about nothing whatsoever, and expresses strong negative judgments toward herself for being a “vegetable.”

[Details of this example are in the 2002 publication cited in bibliography on p. 85. Several pro-symptom positions for depression were found and dissolved. The following account is from her sixth and final session.]

Discovery via symptom deprivation: Therapist prompts her to imagine having real interests; unhurriedly persists with this imaginal focus. Client suddenly exclaims, “I erased myself!” and describes how “my mother takes everything! She fucking takes it all! So I’ve got to erase myself! She always, always, always makes it her accomplishment, not mine. So why should I be anything? So I erased myself, so she couldn’t keep doing that to me.” Client now experiences her blankness as her own solution to her problem of psychological robbery, and recognizes her depression to be an inevitable by-product of living in the blankness that is crucial for safety but makes her future hopelessly empty.

Therapist then continues discovery into why “erasing” herself is the necessary way to be safe: Client brings to light a core presupposition of having no boundaries with mother, a “no walls rule.” With this awareness dawns the possibility of having “walls” so that what she thinks, feels or does remains private and cannot be stolen. She could then safely have interests and accomplishments. This new possibility immediately creates for client the tangible prospect of an appealing future, and she congruently describes rich feelings of excitement and energy.

Outcome: In response to follow-up query two months later, client reported, “It felt like a major breakthrough...this major rage got lifted” and said she had maintained privacy from mother around all significant personal matters. After two years she confirmed that the “black cloud” was gone, she was enthusiastically pursuing a new career, was off antidepressants, and said, “Things are good, in many ways. Things are very good.”

C. Depression expressing unconscious despair, grief, hopelessness

Client: Man with long history of a “drop” into depression every Fall. [This one-session example is video 1097SP, Down Every Year, available online at coherencetherapy.org. For a multi-session example of working with this type of depression, see “Unhappy No Matter What” in DOBT book, pp. 63-90.]

Surfaced emotional reality: At 10 he formed a belief that he failed parents’ expectations so severely that they forever “gave up on me” (he was sent in the Fall from USA to boarding school in Europe, was utterly miserable and begged to come home). Has been in despair ever since, unconsciously.

Outcome: Client subsequently initiated talk with parents about the incident 30 years ago; not once had it been discussed. In this conversation it became real to him that their behavior did not mean they gave up on him, and five months after session reported continuing relief from feeling depressed and inadequate.

If I look at depression as a way of acting / thinking / feeling, then it makes sense that there could be multiple paths to end up that way. Some people could have neurological issues that make it difficult to do otherwise, while others could have the capacity to act/think/feel differently but have settled there as their locally optimal strategy.