Numbers are magic. Windows wanted to differentiate from the tainted 8. They have time before the official release. To my mind, skipping 9 allows them a little more leeway in charging for an upgrade once it comes next year, maybe some fee for going up from 7? Or not.
This is supposed to be the last Windows update of this kind. Who knows how it will work out but in the future there is supposed to be rolling, more frequent, incremental updates. So there isn't to be a Windows 12, if all goes as planned. On the Office side, we subscribe to Office 365 instead of Office 2014. We roll over our subscription. Updates arrive while it remains Office 365. We begin to forget when it first became Office 365 in a way we would not forget when it became Office 2013. I like the idea of continually updated subscription-based Office suite (which I have at home). In work, we are stuck on the aging Office 2007. Subscription pricing is common already with our ancillary software and services. We do have some older legacy, traditionally licensed software and service, and there, the support is dwindling to nonexistent and we are waiting for it to fail and then we will replace it with a subscription product. So if this is the start of a different pricing model, then going up to 10 makes sense, as it has a more epochal ring to it than 9.