Well, GPS is non-trivial. If in an area with radio reflective objects that affects the GPS signals (tall buildings for example). This can cause drop outs, and false readings because of signal delays caused by reflections.
GPS accuracy is maybe 10 meters. If your run has lots of turns, GPS route calculations may end up cutting corners. Also GPS samples are not continuous (battery would die quickly if it was), so when a sample was taken and if you made turns between samples, that could affect the route calculated.
The non-Ultra Apple Watch uses a single GPS radio signal. The Apple Watch Ultra uses an additional GPS radio signal, and gets better accuracy.
If you have your iPhone with you, then the Apple Watch actually uses your iPhones GPS which is even more accurate.
If you have calibrated your standard run as always reporting 8% off, then when you make notes to yourself about your runs, just