Well, I think that traditionally, there are just fewer women in power. I honestly believe that's the only reason. There might be a few instances I think of where women are slightly different from men, but this I highly doubt to be one of them. Even with a more limited number of women in office, I've come across a fair few examples in the Media (in Afrika, off the top of my head, there's at least Winnie Mandela ... )
Men are corruptable due to greed for power, and women are corruptable due to a need for meaning. So you promise a male power broker all control over all the kingdoms of the world, and you promise a female one an environment of total understanding.
So, if they're less corruptible, it's because the corruptors (the competition and peers) aren't used to dealing with them; they try to sell them the default male stuff.