I've recently incorporated 80:20 training into my routine and try to keep within heart rate (reserve) zone 2 for a lot of my runs. I've noticed that often my pace improves throughout the run, getting progressively faster whilst maintaining the same heart rate. Here is a particularly clear example, where my first km was 6:52/km pace, but the last was 6:07/km. On these runs, I am generally trying to run as fast as I can whilst keeping to zone 2 (I occasionally drift briefly in to the bottom of zone 3) as that is more enjoyable. I don't perceive an increase in effort through the run. I was using a chest strap heart monitor, so I think the data is fairly reliable.

I don't think this is particularly problematic, just curious as my intuition would be that my pace would drop as I get more tired?