AeroPod, PowerPod and Newton

When you do a tilt cal typically how long does that cal remain fairly accurate. I calibrate my
iAero and without even riding the bike but coming back an hour later the cal has drifted by as
much as a degree or more as time goes on.The mount is solid and in this example hardly touched.
The calibration happens in the same well marked spot on the garage floor and it always cals to the
same.3/-.3 value???????/

Thanks,
Karl