All of the Garmin models in the discussion so far utilize a barometric altimeter. I've not read Garmin operation manuals, so I don't know the particulars of calibration for these products. I suggest Garmin users read their manuals closely in case user input settings could be the reason for discrepancies. My VDO operation manual states the sensor inlet hole on the cyclocomputer body must be free of obstructions, a lint filled hole would be something to look for, including covering from a cradle mount.

For my barometric altimeter, I calibrate for known base/starting altitude before each ride. I can also recalibrate during the ride when reaching a location with posted surveyed elevation, or elevation derived from a topographical map and committed to memory. Atmospheric pressure will change during a ride regardless of temperature, some rides more than others depending on weather and elapsed time. My barometric altimeter maintains good accuracy for about two mid-day hours on a stable weather day; beyond that it seldom deviates more than 60ft for a long ride or rapid weather change. I don't think calibration is critical for delta totals, but I could be mistaken; I calibrate anyway. If you surveyed the temp reading from five riders at the same time, I'm sure temp readings would all be different.