3% difference speed sensor vs GPS
It may not have to do with sensors or wheel diameter. My gf and I go on rides together both using Strava via phone app. Every time, without fail, the data will claim she rode about .15 per mile more than I, even though we stayed together during the ride. I've tried it with my Garmin and get similar results. I think GPS technology is just jankier than we perceive it to be. I have looked closely at some of the map data after rides and often, my path on the map looks like a squiggly line when I know I wasn't swerving THAT much. Gps is just not perfectly accurate. If your wheel and your dad's wheel are the same diameter and you get different results, this is probably why.