It will dynamically adjust the actual used stride length per step based on the impact that is seen based on the given stride length and weight stats.

So if your mass was more by a decent amount (carrying something?), or the stride length stat is at one end of usable range - it may not have adjusted well to other side of range.

Normally seen with walking stat if someone changes it to their fast exercise pace stride length, and then discovers the grocery store shuffle is way off, as is most of their day.

It's like it can adjust so much, then at end of range it starts losing accuracy.

I've had excellent success, actually shocking the first few times I saw it - picking a stride length from an average pace I'd do running. Which can vary from 7 - 9 / mile depending on amount of hills - which is interesting that Fitbit isn't aware of and the effect of impact is exactly opposite on distance.

And most of my training is jog/walk intervals at longer distances just to take a load off the knees. And yet it'll still get it at 1-3% accuracy or better, like my last long run Fitbit Zip saw 12.17 miles when GPS saw 12.1.

That's incredible in my mind knowing how it estimates.