For some reason I have lower hr zones than a friend I train with. He’s shorter and lighter so you would think he’d burn far less calories. But for some reason he always burns about 200-300 more. He says everything is accurate but it just doesn’t make sense for him to have a higher max and be training at the same average bpm and my calories can be at least 300 less burned than him. Is there anyway I can figure out why this is?