digibud

I have a garmin 500 and my wife uses a Polar Bike computer. We both have the same sized wheel/tires and I've carefully measured the roll out on each bike and they are, as best I can tell, the same. I have set each computer to read a custom wheel size but when we ride together my garmin reports a mile per hour or more faster at 16 or so mph and results in a distance reading longer than my wife's by up to 5% or so. That's reasonably large. I have tried turning gps off and it makes no difference. I can't understand why this would be the case. I'm going to have to triple check the rollout but I've already double checked it. Any thoughts? My best guess is I'm wrong and my roll out length is set too long or hers too short but thought I'd ask if anyone has seen anything similar before.