Credibility is about concreteness. We believe this is happening because the specificity of the details indicates that this is really happening, that only experiencing this situation would lead to such vivid observations.
Read this blog entry about "what it's like being poor" by s/f writer John Scalzi, and I bet it's clear that this is someone who has been poor. We believe this because the details are so precise.
But also imagine that you're writing about a kid who is growing up poor. Notice how many of these precise details could be used to infuse a scene (like the kid getting ready for school) with immediacy (she's getting ready, and her mom is making toast for her, and a roach walks across the bread, and both of them pretend not to notice because they can't afford to throw the bread out).
This is an aspect of voice, I think--the precision of detail. The desire to get so real, to make the fictional scene come alive, that's voice. It's not just the word choice, it's the recognition of the importance of getting the reality in there.