I go back and forth between wanting to say I've got a 50-foot boat, and being happy to save the money on slip/winter storage fees by being able to say I'm a foot shorter. (Just kidding on all this of course.)
Joking aside, there must have been some marketing mojo on this. Our boat is the B49, but in Europe they called it the Oceanis 50. Now here in the states, they've changed from the B49 to the B50. All are the exact same boat (save for the addition of the arch). There must have been an initial view that Americans would be less likely to buy it if it were called a "50," which appears to have changed considering the new branding of the B49 with Arch as the B50. Who knows.