Watch under counting calories during workout

Hello - I recently got a series 4 and have found that it is undercounting calories during my workouts (Orangetheory). I use the HIIT setting and my HR seems to be tracking accurately, but the watch has me burning under 300 total calories and about 225 active for the entire workout (about 55-60 min). In comparison, an outdoor 1.3 mile walk shows me burning 120 calories total. This is just under half the calories of Orangetheory for MUCH lower intensity and about a third of the time, which doesn’t make any sense. Also for comparison, Fitbit and Orangetheory hr monitor would always put me at about 500 total calories for a workout. My personal info is correct, I’ve calibrated multiple times.....insights?