I managed to have a couple of runs outside before all this quarantine stuff started. After that the calibration at the treadmill became a lot better.
However, I am still lost with regards to the outputs.
Treadmill run count:
Treadmill: 10 KM
Suunto App: 12 KM
Google Fit (after sync with Suunto App): 16.69 KM
Before Google Fit I thought that Treadmill count would be the source of truth. However, I am not so sure anymore.
I assume more outside runs should level off the difference, or it could just confirm that treadmill is wrong. But the difference is more or less substantial so far.
One more thing to try is a run with only google fit.