Well sure, maybe it's not suprising that Cameron would take up the post of leader, even if he doesn't believe in the system. But it's still fair game to say that, by his logic, he was elected under an undemocratic system that elected the "loser". (That the later runoff rounds weren't instant doesn't change that point that he lost under the first round of votes, which is the argument that is being made against AV. Unless people would give 1st choices under instant runoff voting differently to non-instant runoff voting, for some reason?)

As for the numbers changing, my guess would this be due to people changing their minds as a result of continued campaigning in the intervening time (at least, I'm having trouble seeing tactical reasons why a David Davis supporter would switch?) It's unclear to me why this system is different in a way that's significant when it comes to the criticisms made against AV?