Was this ride near Indianapolis? The actual elevations there are near 800 feet. It looks like it was low at the start, and corrected during the stop.

My 705 doesn't do corrections once it's started. The elevations around here are 500-600 feet, but sometimes it will show low 300 feet elevations when I turn it on.

A 705 has a Save Location method: Menu-->Save Location -->Average. It starts counting GPS readings and adjusting the calculated elevation. If I let it sit for 60 seconds or so, the elevation reading converges to an accurate number. I hit OK to save it.

Then, if I start a ride within 100 feet of a saved location, it instantly changes the elevation to match the saved location. It won't use any saved locations that I ride past while it's running, only one in range right at the start.

It doesn't look like the 500 has this averaging elevation method, it just has saving a location and entering an elevation number by hand.

Perhaps your 500 will adjust it's elevation if you turn it on and let it sit for 10 or 15 minutes before hitting start. You could test it at home. That's how a GPS should work.

I never seem to be organized enough to turn on my 705 that far in advance, but the few times I've left it for 5 or 10 minutes, it didn't seem to adjust all that much.