I don't know

when I went to the doctor to get a VO2 testing, I had to run on a treadmill for 15 minutes and it was not easy at all. I tracked the calorie burn on my watch to what the programmed and the Apple Watch was 101 calories and their machine had me at 110. That's as accurate as can be or at least I would imagine.
I don't trust the Apple Watch either but I do not believe it is off by 50% on the low side. I think people are overestimating their calorie burn similar to people under estimate how much they eat.
Believe me, I am not doubting you or your testing but I will believe the testing I did in a lab that has my entire measurements and stats.