[codingdave]

Where do you normally live?

I'm totally speculating based on my years living at high altitude, but if you also mostly reside at a higher altitude, you are acclimated to less oxygen, which could decrease your rate when you change to a location with more. If that were the reason, you could expect to see your RHR go back up to "normal" after a few weeks at sea level.

Or it could be something completely unrelated, I just didn't see this in your list of possible causes.