The difference of altitude can not be entirely avoided.
iGS10S primarily calculates elevation data from a barometric altimeter which measures air pressure.
While air pressure typically provides very accurate elevation measurements, it remains subject to factors like seasons, temperature, humidity, climate and wind speed at different time.
So it’s common that the absolute altitude values are different.
The relative values as well as the total ascent, total descent and gradient values of the pneumatic altimeter are accurate, which are more helpful for riding.
As with all barometric altimeters, environment changes can cause elevation readings to "drift," even when stationary or riding at on a level path.
To counter environmental elevation drift, barometric altimeters require regular adjustment.
And you can adjust the altitude manually in iGS618, iGS620 and iGS50E.