Watch calculates steps x step length. Treadmill is based on the rolling of the foot path.

The Apple Watch auto calibrates when you do outdoor running and walking workouts

Indoors? Total toss up. Depends how well each is calibrated. I wouldn’t really worry about it.

There is only 5 seconds between them. Remember one is a watch the other a threasmill. I’d say the watch is more likely to get the time right.

Calibrate your watch

https://support.apple.com/en-us/HT204516