apple watch 6 not tracking miles the same as treadmill

I just got an apple watch series 6 two days ago. I just wore it while on the treadmill and the end result was 0.46 miles less on my watch than on the treadmill. Which is accurate? And how can I get the watch to match the treadmill? How are miles calculated on watch? By arm swinging? So if I hold onto handles, the distance isn't counted correctly? I've read other threads & this seems to be an issue with both indoor & outdoor activities. How disappointing to spend this much money & then have doubts on the accuracy of the data! Please help! Thank you.