Inaccurate Distance Measurements, Even After Calibration Reset

My Apple Watch 5 is consistently under-measuring my runs by 2-5%. I've followed the typical Apple instructions for recalibrating the watch with no improvement. Even when I'm running on a flat and straight route on a clear day with no cloud, building, or tree obstruction for GPS, the distance measurements in the Workout app are short, and are usually often significantly different for the 2 halves of an out-and-back run. For reference, I'm comparing the watch's distance measurement with the known, professionally measured (for race course) distance of my routes. How can I go about correcting or calibrating the watch's GPS (I'm assuming it's a GPS issue since my run should have strong GPS signal throughout and therefore not rely on the accelerometer/pedometer, which I already know is inaccurate and unreliable)?