I think it is a straight calorie burn rate vs effort and therefore fat burn rate?
From experience I know it is hard to follow heart rate bands when on a practical road or trail. I have tried to follow two or three different training regimes in the past with very specific HR zones and it is a nightmare.
My advice would be to look for every opportunity to ride (shops, work commute, local trips) and watch your fat food intake such as biscuits, sweets, cakes, etc.
I have been fighting my weight for 10 years now (I am 44) and I lost nearly a stone when I was in training for a big challenge ride in China but put it nearly all on through Christmas and this winter! Biggest failing I think is having things like donuts, cakes etc which I cannot resist. Some of us only have to look at a donut and we put on pounds!