My guess is that the typical iPhone user who frequents a genius bar is a recent purchaser unfamiliar with the equipment, and there haven't been that many major updates out since they bought their phone. Over time, I'd assume that people would be less likely to need to go to the Apple Store for support. There is also the fact that iPhone sales are growing year-to-year and quarter-to-quarter, further increasing the representation of recent purchasers.
I don't think "50% of people who visit genius bars" is the same as "50% of iPhone users", by a long shot.