Lots of variables. Here in South Orange County, CA, RWGPS overstates the elevation gain as compared to Strava mapping, my Garmin and the bike computers of most everyone I ride with. However in other parts of SoCal, like the San Gabriel mountains, RWGPS elevation seems spot on. You'd think RWGPS and Strava would use the same mapping data, but apparently not, at least here. For example, here's a century I did earlier this month:
RWGPS route planning 97.1 miles, 7,820'
Strava route planning 97.8 miles, 6,730'
My Garmin 520+ recorded 97.9 miles, 6,138'
If you are using a phone app, that's yet another variable. I don't think it's the service provider, smartphones use GPS satellite signals for position (latitude, longitude & elevation).
Go figure.