NPC's and Pets have smaller hitboxes and larger melee range than players
 

When I've been rooted by a pet or when I engage a NPC that roots me they can strike me but I cannot strike them in melee combat. It's as though NPC's have an extra foot or two in reach than a player does.

Currently melee has to be inside the hitbox to actually hit
Currently NPC's can strike you from several yards away (this includes pets)
When you're rooted it's essentially a 18 second stun because you can't even melee the pet down to buy some time.