It's 2 different radio bands, and not necessarily coming from the same transmission tower. So just like 2 music radio station can have different signal quality, so it is with 3G and Non-3G. I speculate that for power savings when iPhone is set to 3G since the radio hardware itself has a higher energy demand, Apple has configured it for lower performance to save on battery. Because I never noticed other 3G phone having this effect. I'm just making all this up.