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.
You are probably right about the AW & accuracy. It samples position data infrequently to conserve battery. That is what causes the overly smoothed GPS courses, and it would affect altitude as well. And as you suspect, this is compounded the faster you are traveling.
Do you mean 735XT? I have this watch, and the altitude is often off by quite a bit, especially off road, because of no altimeter.
I'm seriously considering the Pace 2, but it doesn't have a trail feature. I like trail running a bit, but not really sure if that's much of an issue. Is this related to GPS accuracy when under tree cover?
Doh-typo. Yes, I meant the 735XT. Mine was generally good on runs, but I always ran on roads with very good GPS coverage. The worse your GPS (trails & trees) the worse altitude would be.