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.

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

Calibrate your watch

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