Guess what! The American edition is not identical to the British edition. Does this happen often?

Yes, more often than you'd think! Actually, many years ago when I was reading the Horatio Hornblower series for the first time, the paperback edition of one book, published in NY, ended, I thought, rather oddly. Eventually I found a copy of the British edition, which, indeed, had another chapter. I wrote to the British publishers to draw this to their attention, and their answer was more or less that this all happened so long ago that nobody in the company knew anything about it anymore...dunno if it was ever fixed.
For another eg, apparently "Harry Potter and the Philosopher's Stone" was published in the US as "Harry Potter and the Sorcerer's Stone" on the dubious grounds that American children wouldn't know what the Philosopher's Stone was. (Gimme a break!)Does anyone know if that's true, or just an urban myth?