My iPhone XR frequently miscounts steps compared to spouses' iPhone X

My spouse and I both moved to iPhone X recently, she has the X and I have an XR. But we discovered a frustrating anomaly; my XR regularly under counts my steps when we both walk the same route.

I'm not talking about the difference of a few hundred steps, due the various stride length. I'm talking a full 50% fewer steps, the difference of a 1.6 mile walk, while my spouse has the 3.2 miles walk, and we walked together! It makes no difference which third-party app I use, it's always off. Frankly, the Apple health app is useless as it only gives you steps not mileage.

It must be something wrong in the hardware that "notices" the step movement of the iPhone XR while on my person. I thought having "open apps" might be the cause, but closing all apps made no difference. When we're trying to be faithful in our exercise, and record our movement for health purposes, it is so frustrating when the data I receive is unreliable, and it happens almost every week--sometimes more than once.

I guess it's time to invest in a Fitbit and forget the Apple health stuff. Too bad, though, since the phone is already integrated--my XR model just can't count reliably.