It's an estimate, probably something similar to what KBB or NADA does where you put the year/make/model and inputs the mileage, that's the estimated. My PSD only has 110k on it, but KBB says it should have 161k.
Edit - looks like KBB changed their site and they don't do that anymore.
Edit - looks like KBB changed their site and they don't do that anymore.