Players can jump over fences when there's carpet on top of them, but mobs can't. I know that mob ai's only see "block" or "no block". This makes them see paths that they can't go and not see paths they can go many times, but in this case it could be fixed without changing the whole pathfinding system. Fences are probably set as "block and another block on top", because mobs can't go over it normally. My idea is to make fences be seen (for the mobs) as "block and another block on top, if the block on top isn't carpet or something like that". That would be an easy fix for this mob behaviour, the mobs would see the fence as block, but the block above as empty, so they would jump and be on the correct height to go over the carpet.