Inconsistent Hostile MOB Spawning on Upper Half Blocks

Hostile MOB spawn inconsistently on half slab upper cobblestone. I built a 30x30x3 (interior dimension) cobblestone structure. The walls and ceiling are cobblestone; the floor is cobblestone slab, upper half. When I stood atop the structure, roughly at the center, for one night, three skeletons spawned inside. When I stood inside the structure, roughly in the center, for five days, nothing spawned.