Not sure there is much concrete to be gathered here. At a high level, I think you anaerobic capacity is allowing you to get closer to your max heart rate, which others have touched on. Other than that your ability to endure discomfort has probably improved. Most people don’t realize what going hard means until they’ve done some of these efforts. Last, heart rate is kind of a fickle thing. I’ve had ramp tests that were bad, where I hit my max heart rate and I’ve had ramp tests that were good where I didn’t. Other than max heart rate, I do think it is somewhat helpful to pay attention to the heart rate you can sustain for a longer period of time. Typically called your threshold heartrate. You may notice you used to fade in the high 160’s, but now you can handled the low 170’s. Overall, I think that’s a good sign, but being heart rate is affected by so many other things, isn’t really a reliable indicator.