why some of the cards play less than others?
I suspect that all you are seeing is the natural result of random selection.
With random selection there will be some cards that just happen to be selected more often than others and this will be by pure chance, not because of some bias favouring those cards.
If you want each card to be played the same number of times but in a random order then you don't want a random selection mechanism, what you want is a random shuffle. However, iStripper does not have such an option though i think it might be good if it did even if this was only as a way of playing a loaded playlist.