Thanks SE. From reading that (without going to read MMR's site), I would gather that they use different correction methods, and have different thresholds for recording something as elevation gain.
Strava:
"How do you calculate elevation gain?
Elevation data on Strava is smoothed to take out noise— we have a 'threshold' where climbing needs to occur consistently for more than 10 meters before it is added to the total elevation gain. If we did not have this threshold, the elevation numbers would be inflated for longer activities. This gain threshold applies for both rides and runs."