My garmin seems to overstate elevation changes so I end up with double the "climbing" each ride that others see. That definitely skews power estimates.
I've been advised that my garmin mounting should be changed to be more level (parallel to road surface). It seems to affect the barometric pressure readings used to calculate elevation.
Barometric pressure also will change for a given location as weather changes. I've had many rides where the elevation of my parked car changed 50 feet between ride start and the finish an hour later.

I upload my rides to strava to get the analysis. I gave up on the phone app so I could get the ant+ cadence and HRM data and save the phone battery for phonecalls.