This may be a stupid question, but...

There're a couple of psych effects we have evidence for. Specifically, we have evidence for a sort of consistency effect. For example (relevant to my question) there's apparently evidence for stuff like if someone ends up tending to do small favors for others or being nice to them/etc, they'll be willing to continue to do so, more easily willing to do bigger things later.

And there's also willpower/niceness "used up"ness effects, whereby apparently (as I understand it), one might do one nice thing then, feeling they "filled up their virtue quota" be nasty elsewhere. (ie, apparently one of the less obvious dangers of religion is you, say, go to church or whatever, and thus later in the day you don't even bother tipping (or tip poorly) when you go to a restaurant because you're "already virtuous" and thus don't have to do any more.)

How is it that we can simultaneously have evidence of these two things when they directly contradict each other? Or am I being totally stupid here?

(EDIT: just to clarify, I meant I was asking "How can it be that the sum total of evidence support both these positions when they seem to me to directly contradict each other?")