I ran into this a few years ago when I had a Giro helmet and my son couldn't get a good fit in a Giro and ended up with a Bell, which is a little odd since Giro and Bell had the same ownership. Offhand, Giro, Louis Garneau and Specialized fit my more or less oval head, Bell and Cannondale for his rounder head.
I've seen the same thing with motorcycle helmets, Shoei fit me perfectly, Arai needed a size larger and AGV and Bieffe wouldn't fit me at all.