I think they've learned from the Office 2007 metrics and feedback. Also they have listed the reasons for choosing the ribbon over other approaches. I don't see a 'jump'.

>And what user data prompted them to create 203 buttons if some of the (far less than 200) commands they have now don't get used in more than 1% of user sessions?

You say that as if the 200 buttons were on one screen. They are spread out over different contexts and tabs. I believe they were trying to make the commands more discoverable from the previous versions.