Mobs cannot spawn at a given X, Y, Z coordinate if there is a solid block in the -X or -Z direction. This is likely because the game attempts to spawn the mob at the bottom-north-west corner of the block and therefore detects collisions with blocks to the north or west. As a consequence, no mobs can spawn at all on southern or eastern slopes that are 45 degrees or steeper. This makes for an unnatural distribution of mobs in worlds, with barren zones and overly dense zones. It also limits the possibilties for mob farms and/or limits farm efficiency.

Steps to reproduce

1. Open the attached test world.
2. Over one hill and then the other, fly up between the command blocks until you hit the glass, then look down and observe spawns.
3. Use the command blocks to alternate day and night, kill all mobs, or send up signals to help locate the mobs (especially in the water).
4. Note where mobs spawn over several trials. The red glass marks north.

Expected result

Mobs spawn all over each hill with uniform distribution.

Actual result

Mobs only spawn on the north and west slopes.