the watch doesn't match my workout using a treadmill, it gets less miles than treadmill says

My watch is always wildly inaccurate when I run on the treadmill no matter how many times I calibrate it. I fixed this by purchasing Stryd, a pod you put on your sneakers that pairs to your watch via Bluetooth. Now it’s scarily precise every time. It sucks to have to spend more money to make the Apple Watch usable as a running watch but I’m so much happier now that I did. Sharing in case you’re also frustrated enough to spend some money to fix it. Apple needs to come out with their own pod.

When you are on the treadmill, allow the arm with your Apple Watch to swing naturally.  Holding onto a handrail, for instance, will result in less miles than actually walked on the treadmill. For more info, see the "Make sure that you earn Move and Exercise credit" section in:

Get the most accurate measurements using your Apple Watch - Apple Support

You should also make sure your watch is calibrated:

Calibrating your Apple Watch for improved Workout and Activity accuracy - Apple Support

Also, see the solution to:

apple watch series 5 activity recording n… - Apple Community