Discrepancy in how Apple Watch compares Walking Speed data with Walking Exercise data

I work out by walking on a treadmill in the basement of my apartment building. Yesterday my Apple Watch Series 10 recorded that I'd walked 33 min and gone 2 miles. A little arithmetic shows that if you go 2 miles in 33 minutes you're walking over 3.6 MPH. But for Walking Speed my Apple watch says that my range yesterday was 1.8-2.8 MPH. This is a large discrepancy -- 2.8 is about 22% less than 3.6.

I realize I can recalibrate my Apple Watch, but the recalibration appears to require walking *outdoors* for 20 minutes and using the Watch's GPS. (Note: the Watch has always had accurate details about my age, height and weight.) And in any case the recalibration seems unnecessary; if the Watch accurately recorded my treadmill session it knows I averaged 3.6 MPH during the workout, so it ought to use 3.6 MPH as the upper limit of my Walking Speed -- not 2.8 MPH.

Is there any way to minimize the discrepancy between how the Watch displays Walking Speed data and Walking Exercise data? I love all the data my Apple Watch gives me, but this glitch baffles me.