dont know.
but here is some related info from a different perspective.
this info relates to elevation change (not necessarily correct indication of initial elevation itself).
can you co-relate these elevation inconsistencies to atmospheric barometric pressure changing at a quick pace outside of your building in your area?
when i go running in the hills around my house, on days when the weather conditions are changing quickly, the equivalent flights that i run up drops dramatically from the usual (and verified) 28-30 equivalent flights to only around 20-21 equivalent flights, running the same course.
also there is this: the equivalent flights data is different between what your iPhone is recording, and what your Watch is recording
in the iPhone Health app you can selectively re-order and turn off either the Watch or the iPhone writing of this data, and by doing this you can actually show that the data that the Watch and the iPhone are writing about elevation are different from each other.
in my case, my iPhone provides better elevation change data than the Watch.
in apple support documents apple does indicate the elevation level itself is +/- 20m.
