Despite Fitbit calling it restingHR - it's not the traditional meaning of the term.

Normally that means first thing in morning after waking, and settling down after alarm freak out.

Fitbit seems to measure your lows and average them.

Still a good result though no matter.

Possible reasons for lowering -
Body is making some improvements from exercise that had never really been done prior, and those are always bigger at the start.
Something in your diet changed that was elevating it beyond what body really needed, now it's lowering more to where it's needed.
Accuracy of reading HR is improving or getting worse, so it was wrong and is now more right, or vice-versa. You ever felt pulse and measured to compare at sitting, standing, moving a little, ect to confirm Fitbit is reading correctly?

The getting fit seems to fit in better with being in lower HR zones though, but all three could cause range changes like that too.
But when you get fit that is what happens - heart doesn't need to beat as much to provided needed oxygen for a certain level of effort.
So your daily activity is burning the same calories if moving the same mass in the same way - it's just more fat calorie now then carb calories because you are providing enough oxygen with less heart beats.

Probably should notice the same thing with exercise, harder to push the HR as high as it once was.
So the muscles will still get the same workout, except for the muscle of the heart.

And in fact that is the problem with walking as only form of exercise - you reach a certain fitness level - and then it's no longer a stress on the body to improve, unless you walk faster, or hills, or with extra weight.
And if you lose weight and don't do one of those things - it actually becomes easier for your body and you'll lose fitness level moving around less mass.

So keep it up - but now to keep working out your heard - you'll need to step it up too.

Then your restingHR will probably keep on moving down.

Oh, it sounds like the vast amount of that time was in the "fat-burning" zone in daily activity. Well, most your daily activity has major fuel source as fat anyway (except right after meals), so even though HR isn't getting that high - you are burning more fat now, less carbs, with that effort of time.

Ah, so perhaps they decided they can figure out what the before wakeup HR is, and is correctly using that. I think they already had it and used it for the HRM calorie burn estimate.

It still might be useful to then base a daily average low HR on that though.

I could see that useful to inform you how much higher your daily HR is compared to restingHR - perhaps give a clue if you have stress issues, or perhaps caffeine issues, ect, that keeps you elevated all day.