Since you were over 1 mph faster on the recent rides with less average HR. I'd put more to the idea your cardio vascular system is in much better shape and your heart pumping a larger volume of blood each beat.
If your average speed for the same HR was declining, then you might worry, but it's probably not because of your max HR declining. If it is, you'll probably be 15 or 20 years older than your are now.