Why all OS X verions are 10?
OS X is supposed to be pronounced "Oh Ess Ten" though no-one outside Apple ever seems to, common parlance would be "oss ex".

Though OS XI would, I guess, be OS eleven, how many people are going to end up calling it "oss exie" or "oss ex eye"?

The actual answer to the question "Why?" is presumably because Steve Jobs decided it would be; &amp; that would brook no argument.