I don't think Garmin worries much about their software accuracy. For example, they could recalibrate the elevation when the bike was stopped, but my 705 never adjusts once I hit Start.

I rode in gusty winds today, and the 705 showed 2000 feet of elevation, when ridewithgps calculates 1100 feet for the route. Each sudden gust affects the 705's barometer. But, usually the elevation is quite accurate. I've done rides with lots of up and down climbing on the Blue Ridge Parkway, and the 705 is often within 20-30 feet of the posted elevation at the overlooks.

All these GPS recordings need smoothing of the raw data, and each upload site or software does it's own methods. I noted some differences in a thread post from 2014.