I have never, EVER ridden with someone who was using the phone app and had them end up with less distance or elevation. Last time out, the person using the phone app had the most distance, and I knew for an absolute fact I had ridden about 1 mile further, as I had ridden ahead and looped back several times, and made several parking lot loops waiting for others to catch up. Now I can't 100% say it's the phone or the GPS-- as the apps like Strava and RWGPS basically use their own math.

Phone GPS latency also helps guys gobble up short-segment KOMs-- the ones on Strava where their average speed is higher than their max speed.