Fenix 5 Treadmill Calibration

Hello people!

New to this forum as I am new to Garmin and athletic watches in general. I run 3 weeks ago for the first time the Athens Authentic Marathon, did quite well, so I decided to buy a watch to help me improve for next year.

So here we go again with another question on treadmill calibration.

Obviously all treadmills themselves are Calibrated differently. This is a fact.

I run on a treadmill, then I calibrate. Then I run outside and when I go again on the same treadmill it’s off quite a bit.

I run on one treadmill then I calibrate, when I run on another treadmill it’s off.

All measurements have a difference of 5% to 8%

The only time they are consistent are if I run two times in a row on the same treadmill, with same shoes, the same pace etc. then both measurements are identical.

Questions:

1) Is the above behavior normal?
2) If it is, which measure of all should I trust?
2) what sensors/ movements does the watch use to assume the distance I run?
3) is the treadmill calibration altered by the next run I will do outdoors?
4) what can I do to calibrate the watch the most accurate way as it is impossible every time after a run to calibrate the watch.

Thanks in advance
Michalis