Zombies seem to have an area around them that they can attack in. This area seems to be bigger than their bounding box... which means zombies can attack through corners where their "attack area" goes through the blocks their bounding box is right up against, as long as the corner goes through a 1-wide diagonal block placement.
This is especially annoying when you have a lot of villagers which like to group up in such a corner of a home-made house for some reason, especially during a zombie siege.

This might (and probably does) apply to other melee-attacking mobs too. I've done my best to search for this issue, but I've been unable to find this issue reported.