Why doesn't the GPS distance match the distance tracked on my stride sensor or speed/cadence sensor?

GPS and sensor data should be very close, but if there is a discrepancy the sensor data is the most accurate. The GPS function on the iPhone can detect your location +/- 10 meters and then determines distance traveled based on those numbers. Additionally, when trees, clouds and limited data coverage come into play, you can see wrong GPS information or have cut outs in GPS coverage. So, you can definitely expect some discrepancies between those two values in the export.