With my Apple Watch (altimeter), the Strava upload consistently reports about 30% more climbing than the workout on the watch. I assume Strava is more accurate because it's referencing some geospatial database. The interesting thing is this doesn't happen on runs, watch and Strava agree very closely.

I think what's happening on rides are that the altimeter doesn't sample very often and at riding speeds it's missing a lot of quick changes in elevation, but running is slow enough that it gets pretty close. I guess I could test this by doing a very slow ride and seeing if the agreement is better.