The answer is pretty plain: The Office division cut the team that was supposed to ship the Metro-styled Office apps for 2013 despite the Windows division objection. Windows then relented and added 'desktop mode' for ARM just to keep Office in the release. Office tweaked the existing UI to make buttons 'bigger.' The original plan called for the Metro UI to be available whenever a user interacted with his hands -- Surface was the primary objective, but Office 2013 should work great on the Toshiba tablet as well.

The decision to cut Metro support was made in Q1 2011 and distributed to all Office managers by Kurt himself. The team that was working on Metro (called Modern Office eXperience, MOX) shrunk and merged with OneNote. Hence, why we see OneNote has Metro and no other app does. The reasoning was "the Win8 APIs are not stable enough for us to finish on schedule with the team we have. We either ship 10/11/12 or slip by a year to support Windows 8." They decided to ship (RTM) on 10/11/12. We should see a 'surprise' SP1 that supports Metro and iOS by the end of this year (this is pure speculation from me -- I have no NDA knowledge about plans past 10/11/12).

You are seeing a classical "ship the orgchart" dilema from Microsoft with Office and Windows this season. It's sad, really, but these tensions are why I don't work there.