I didn't see a lot about this online. However, I've read before that cyclists in the Tour de France see their heart rates drop as the race progresses. I think you might just be seeing the junior version of this on single rides.
I have noticed the same thing with my heart rate, but it happens well before 50 miles. My thought is that's it happens because, over time, my body acts more efficiently, so that my heart doesn't need to pump as much oxygen through my body.
In any event, a seven point drop in average heart rate over 100 miles doesn't seem like anything to worry about.
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