The hitbox of a normal grass block and the farmland block are the same, but the farmland block is slightly smaller (just as the path block), so there is a slight offset. I'm not sure if this is a bug or just intended but it looks wrong.
MC-85162
Slight player/mob offset when on Farmland Block
Resolved
Fixed
3
Confirmed
Minecraft 15w32a, Minecraft 15w32c, Minecraft 15w43c, Minecraft 15w44a, Minecraft 15w45a, , Minecraft 1.9.1 Pre-Release 1, Minecraft 1.9.1 Pre-Release 2, Minecraft 1.9.1 Pre-Release 3, Minecraft 1.9.4, Minecraft 16w20a
Minecraft 1.10.1
Retrieved
Confirmed. Does not happen on grass path.