Mobs underwater in minecarts can drown

If a mob is underwater in a minecart, they can drown. This seems odd to me since while a mob is "sitting" in a minecart, they only take up one block high of space, and the minecart is one block of space, and unless the water fills up the minecart there should be a half or 5/16 block of air above the minecart which would allow the mob to breath and not drown