Altitude is a tricky thing with these kinds of devices. They’re usually based on pressure readings from a sensor inside the device but that reading can be affected greatly by a number of factors.
With all the people riding around with these sensors now, shouldn't it be possible to synthesize a very accurate basemap elevation from the reported barometric and GPS readings?
Strava segments in my area often have very wrong altitude data. Chicago is extremely flat yet there are segments that report gradients of over 10%, sometimes as much as 27%. I believe they are thrown off by buildings and the frequent elevated train tracks.