I don't agree that Karrubi poor performance is strange. He was doing poorly in all the polls leading up to this election.
I went and looked again at why I miscalculated the outcome of this election. I did so assuming there was no fraud, looking at the only scientific poll taken before the election. Once I looked closely again at what that poll indicated, frankly the result is not all that surprising. As long as you accept one lesson from this election: that the whole "boycot" or "not boycot" the election crowd have marginal influence with merely a few million people listening to them. Otherwise, once you accept that premise, the results totally fit the picture that was shown through the poll by Terror Free Tomorrow, where Ahmadinejad started with almost 16 million committed voters whereas Mousavi had only 6.5 million. The majority of the undecided, who constituted 12.5 million, apparently chose Ahmadinejad while only around 40-45% of the undecided chose Mousavi. A million or so voters who had indicated they would not vote ended up voting, presumably all of them for Mousavi. (In that poll, voter turnout was projected at 82% while the actual turnout was 85%).