Inconsitant recordings

I have recently been using my Apple Watch to record my runs, it’s a non gps so connects through my phone.

The last 3 runs I have binned the watch and just used my phone. This has ended up recording slightly greater distance of about 200mtrs but roughly the same time resulting in faster times! (Under 20min 5k so I’m not complaining)

Went back to the watch tonight and again it was 200m shorter

Any clues why this would be considering the watch is still using the phone for gps location?