My point is that if my average HR is much higher in one HIIT workout than another, I will burn more calories in that workout. Hence, I should be higher on the burn bar in that workout. But I am always in the middle of the burn bar regardless.
Ok, but you will only be higher if you are burning more calories than other people that have set the 'front of the pack' level. At the end of each workout, it displays your position in terms of calories burned and where you would have a need to be in terms of being higher.
So below, for example, I was in front of the pack, that is determined by burning more than 279 calories, I got to 288.
Have a look at where your position was in previous workouts and check it in future ones.
For clarity, there are 5 positions. Using that image above.
Behind the pack = less than 60
In the pack = 60 - 88
Middle of the pack = 88 - 279
Front of the pack = 279 - 326
Ahead of the pack = more than 326
Also note that this changes frequently, I did the above workout 4 days ago, I did the same workout earlier today, to be front of the pack today the burn has increased, you need to be 282 - 328. As more people do the workout their burn is taken into account and the calorie burn changed.
And of course, no two workouts are the same in terms of burn bar, the ranges will be different in each.