According an FAQ on the SuperSite for Windows:

Office 14 is the next version of Office 2007 (or what Microsoft calls the 2007 Microsoft Office System). […] 14 is the version number. Office 2007 was called Office 12 internally at Microsoft. The company skipped 13 for superstitious reasons.

Superstitious silliness if you ask me. I did wonder though how they were going to handle this, but I didn’t think they’d actually skip the version number!