How does the treadmill mode calculate distance?

[Bofetrebof]

In my Suunto 7, I usually use the treadmill mode when going for a leasure walk. Strangely, I don’t see a consistent correlation between the number of steps and the distance displayed.
I would assume there should be a step length configuration somewhere, but I didn’t find it.

So is there a step length configuration and where?

Second, is distance calculated from other factors than step length?
Thanks