Rabbits spawning underground

I have rabbits spawning in an abandoned mineshaft, ~30 blocks under the surface, in ice plains/cold taiga biome. As I understand, rabbits should only be able to spawn on grass, snow or sand. The floors is mostly stone, but there are some dirt patches (no grass though). The fences ,rails & cobwebs have been harvested, so (explored part of) the mineshaft is currently just a set of well lit 3x3 corridors.