I have a feeling, from all of the SP that I've played, that there is a weighting system to the heroes. Such that you have the base 0-6% chance to recieve a hero in any given turn, but then you do another roll against your list of available heroes, and some will be much more common than others.
Anyway, I know about small sample streaks and trends in RNGs, but it still seems this may be the case.
