9 doesn't have to come before 10. In the realm of version numbering, you can do pretty much anything you want. Minecraft usually jumps around with things like 1.6 to 1.6.4 without anything in between.
one rumor I heard of is since in some counter's *cough* japan *cough* 9 is an unlucky number so the decided to use 10.
more reasons:
apparently there is script errors with it being called windows 9 involving windows 1995 and windows 1999.
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