IPhone Health App Distance (In)accuracy

Hello all. This is something I've struggled with literally for years. The question has been asked and answered dozens of times on various forums, but the answers given are contradictory. It comes back to this: How is distance calculated in the health app on the iPhone? Apparently knowledgeable people have claimed that it uses GPS. Other apparently knowledgeable people have claimed that it uses steps and other date (height?) to estimate step size.

My wife and I are in Paris. We have identical iPhones. We're doing a lot of walking. We go on long walks together, never leaving each other's side. Her distance estimates are typically at least 25% higher than mine.

I've just compared settings. We both have location services turned on. Under location services/system services, we both have "motion calibration and distance" turned on. The only difference I can find in settings is that inside the health app, mine indicates that it's using both "Health" and "iPhone" as data sources under "Walking + Running Distance." She has only "iPhone" listed. It does not seem to be possible to delete "Health" from mine, and it isn't clear how I would (or that I should) add "Health" to hers.

As I said, we are in Paris. The buildings here are not too tall. Plus, we're together. If the Health app uses GPS, as some have claimed, we should not be getting such different numbers.

Any facts to offer, or well-informed opinions? Thanks in advance.

Jim