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Yes! I think this is it. The wikipedia article links to these ray diagrams, which I found helpful (particularly the fourth picture).
I suspected it had to do with an overlap in the penumbra, or the "fuzzy edges", of the shadow, but I kept getting confused because the observation isn't what you would expect, if you think of the penumbra as two separate pictures that you're simply "adding together" as they overlap.
See also this highly-upvoted question on the Physics Stack Exchange, which deals with your question.