I can think of two drivers who deserved it more than Button.
Lewis Hamilton and Dario Franchetti.
Not sure how Button won it.
When I heard this, I was surprised (and that's probably an understatement)...
However, how does this sound as an explanation; There may have been four types of voters here;
1. People who voted for Button as they were either Button fans, or genuinely thought he'd been the best performer.
2. People who voted for Lewis Hamilton in international driver of the year, but wanted to vote for a different driver for international driver of the year
3. People who voted for Lewis Hamilton in international driver, and also voted for him as British driver
4. People who voted for Vettel (or Alonso) in international, but then voted for Hamilton in British driver
1+2>3+4.....