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.
Sounds like a great reason to introduce new hardware in the fall that takes advantage of the new device setup process and wireless syncing to iTunes/iCloud.
They'll be using it once they've been to the Genius bar. Bear in mind that some of those 50% will have been repeat offenders (some of the other 50% too since they'll have had their phone updated only at the Genius bar).
