I would be similarly skeptical, if it were not for the fact that mine is so consistently low. And that's with going and plotting out the route post-ride in Strava, Google Maps, etc. I rode side-by-side for 100 miles with my wife, and at the end she had 2,800ft, and I had 2,300. Strava corrected it to 2,851ft. Her 500 has been exceptionally consistent in terms of both elevation and distance. The 520, with a speed sensor, will give different mileage when riding the same lap of a route twice. Along with the elevation discrepancies, it has consistently shorted me around 3% in distance relative to my wife's 500. I have since done away with using the auto-calibration for rollout, going with a physical measurement. Nevertheless, the Karoo and the Garmin, paired to the same sensor and programmed for the exact same rollout, the 520 still reads less distance. My last two rides with my wife:

Her Edge 500: 21.16 miles, 440 feet
My Karoo: 21.18 miles, 452 feet
My 520: 21.05 miles, 417 feet

Her Edge 500: 27.64 miles, 758 feet
My Karoo: 27.33 miles, 778 feet
My 520: 26.69 miles, 686 feet

Again, it wouldn't matter much (as if it matters much now) if I didn't put in ~10k miles every year. I estimate in the time I've had the 520, it's shorted me by more than an average person rides in an entire year.