Breathing can make a difference?

Before my surgery a little over six weeks ago I was a mouth breather for a long time. I could not breath through my nose and get enough oxygen. The surgery opened up the airways and now (after some intentional practice!) I can breath through my nose. I'm not sure what the reasoning behind it is, because I still mainly keep my mouth open when I bike, but it has affected my heart rate during rides.

If I toss out my first 3 rides after the surgery, where I was still really healing, my max and average heart is around 8-10 beats lower for the last five weeks during rides than the two months before the surgery. Doing the same rides and even some that are more difficult. My distance and speed have also increased after the surgery. Of course, some of this makes sense with just getting more fit throughout the year, but I find that I am having difficulty even getting close to some of the heart rate maxes and averages that I could do before the surgery.

This is just weird to me. Anyone have any insights?