Right, but the problem with having the narrow part on the inside is that any arrow that hits within the "cone" will be "funneled" right into the defender; where

Right, but the problem with having the narrow part on the inside is that any arrow that hits within the "cone" will be "funneled" right into the defender; whereas what you want is for attackers' arrows to hit only if they manage to get just the right place. The wording is slightly unclear, but I can't imagine anyone this well-versed in medieval warfare wouldn't know that arrow slits would be wider inside than outside.