Treadmill distance way off, calibration making things even worse.

So, of course, you can never know if a treadmill is counting the distance perfectly. But the ones I've been using for years have reported distances consistantly, and on par with my outdoor running distance/time. And my FR 235 did a good job estimating Treadmill distance, consistently reporting around 0,95 km per 1 km on the teadmill.

The FR 945 is a completely different story. It started of recording about 1,4 km per 1 km on the treadmill on my standard 5 km @ 4:30 min/km. Using the Calibrate after saving function doesn't make things any better, actually they are getting worse! Today (fw 2.80 on the watch) every 1 km on the treadmill was more than 2 km on the watch, resulting in a world record over 10k at 21:19 min - WOHOOO!  

As I understand the concept of calibrating, the watch should take the distance I put in when calibrating, and use it together with the number of steps measured for the same workout to get a new stride length for better treadmill distance calculation in the future? Or does the calibration feature only edit the distance for that specific run? 

The only setting that I've found that could mess things up is the optional manual stride length for walking in Garmin connect app, but I have that one of, and it shouldnt be relevant for tredmill running nayway?

This problem seems widesperad on the FR245, is it equally bad for everyone on FR 945?