But it's close enough. My wife and I use our watches to track our walks for exercise. There are slightly different results, I think that is due to the different stride measurements between us. You can set the measure of your stride from one foot to the other, for walking or running. We're seniors in our 70's and got the watches for health benefits, but like all the various other functions.
My Apple watch logs about 1 in 5 trips (20%) up the stairs per day. Turns out there are quite a few complaints about inaccurate tracking flights of stairs.