Improving fitness, as you workout more your body adapts making itself more capable. I believe if you REALLY needed to tap into that extra 10 or 5 bpm (ie a bear is chasing you OR you were in a sprint for the top podium spot) your body would find it. When I’m free riding or training I have a hard time getting my HR past a certain level, but when I’m racing I find myself working “fairly comfortably” in those near-top HR zones.