They spawn in the darkness I suppose, and aside from the supports the entire mine is atleast 3 blocks tall. It'd make sense they spawn in there, but I think there's a bit of code that makes them spawn in there more often than other places.
I think they spawn equally everywhere because I've been finding a lot in wide open spaces like deserts and plains.
Also,
lovey waterfall
More oddly shaped terrain...
Whats wrong with the generator? D: