At least historically, ridewithgps used much more detailed topo maps than mapmyride. I'm not sure whether that's still the case. So very often, short steep hills like we have in the eastern US wouldn't show up on mapmyride, and a longer climb would register as a long shallow climb, instead of the reality of a series of 20% climbs and slightly shorter descents. A century with about 10,000 feet of climbing, that tracked fairly accurately on ridewithgps, would show up as about 2,500 feet on mapmyride, hardly "off a few feet."