We both have an old iphone 5 and were together for all of the rides, what's going on?
Are you both using the Strava ios app? Try experimenting with another activity tracker. Is you iPhone running with the low power option on? It might reduce the number of GPS tracks logged?
I have now looked through all the settings on both phones and checked they are the same, not tried any other activity tracker and not sure if there's anything that will work with iphone 5.
Can you check the distance/duration of the individual rides?
My guess is that one phone was left running for a bit and accrued distance due to jitter being sat in a pocket/bag/whatever whilst at a cafe or back home.
Are you both using the Strava ios app? Try experimenting with another activity tracker. Is you iPhone running with the low power option on? It might reduce the number of GPS tracks logged?