For all vanilla mob heads except Dragon Heads, 1 pixel on their texture is the same scale as a pixel on a block (1/16 of a block). However, 1 pixel on a placed Chimpanzee Head is slightly larger than 1/16 of a block, which causes their model to appear at a slightly larger scale than other mob heads and to not line up with the texture of the block they are placed on.

Additionally, Chimpanzee Heads placed on the side of a block (chimpanzee_wall_head) do not render touching the block they are placed against.

Chimpanzee Head:

Piglin Head:

Chimpanzee Wall Head: