Beepster
Apparently they skipped over 9 because it had the potential of screwing up programs. Essentially they were worried that certain programs would see the 9 and think they were dealing with win 95 or 98.
So nothing like how Word went from Word 2 to Word 6 with no Word 3, 4 or 5 then. Which, of course, had nothing to do with Wordperfect being at version 5 when Word 6 was released.
Nothing to do with not wanting to be a version number behind OS X or the same as the soon to be released iOS 9 either.
MS have played marketing games with version numbers for a long time. It seems the less technically aware customer might be persuaded that if product X is at version 10 it must be an "upgrade" compared to product Y v.9 even if they are two different applications made by two different companies,