Apple Watch GPS Mystery

Riddle me this, Apple: how does your Apple Watch Series 3 Nike Plus GPS lose me on a university 400 m track? I kept getting mile notices at wildly different points on this track, and I only diverted once during the second mile (by about 15 m to get water). I thought maybe I was going crazy but then I saw the data broken down by a third party app with mile markers (see photo).

Do I need to figure out how to calibrate it? I’ve been running heavy mileage with my AW and iPhone for 2 weeks almost every day, and have my “Motion Calibration & Distance” feature turned on on my iPhone.

This is incredibly frustrating for a runner in the middle of marathon training. I want to go back to Garmin watches.