Running distance on watch seems to be way off

So I go on runs with my girlfriend and I use my watch to track and she uses Strava, we literally run side by side so we know we’re going the exact same distance, but our measurements will be so far off. Today my watch clocked in at 1.66 miles while her strava clocked it at 2.2 miles. I then put the start and end points into google maps and google maps had it at about 1 mile each way, so 2 miles total. It makes sense to me that Strava (just on phone) would be slightly off, but why would my watch be almost half a mile off??

I was doing walk and run intervals, would this have anything to do with it, that it measured the time i was actually running vs the time I was walking? If so, how did it do that because the map is continuous.

how do I calibrate it to be better and is it likely off in its other measurements (walking and swimming)??