Is Overbuff Accurate?

I'm looking at some of my stats and the data on Overbuff is fascinating. They have everything broken down so I can instantly tell where I'm average, above average, or below average for certain metrics. My only concern is that it's too good to be true. Are they able to accurately get all the stats from Blizzards servers? If so, how? And does it take ALL players' stats even if they never visit Overbuff?