It's also why the same wrapper is called twenty different things - Habano, Criollo, Habana-seed...
To further illustrate as of January 2009, Habanos, S.A. has over 72 parejo vitolas and 11 figurado vitolas. The difference between some is something as simple as a pigtail cap. And I'm sure they're more as every single regional release and EL seems to be a new size. As most have historically followed the Cubans, you can see why things get dicey. Perelman has simplified it in his CigarCyclopedia books. But even then there is some range and cigars that don't fit anything listed...
And another way to look at it is with cars. Just because it's a coupe doesn't mean every car manufacturer will name it "Coupe".