Edmunds bases its True Market Value (TMV) estimations on actual sales transaction data of similarly-equipped vehicles in the same region. On top of using actual sales, we take into account factors such as a vehicle's location, color, mileage, condition and options to help deliver a more accurate estimation of the vehicle's value.
Unfortunately, we can't speak for how KBB, NADA, and other sites calculate their values. Most likely, the difference in estimations come from the algorithm each company uses to compute their values and the data they're basing the calculations off of.