Same as unique car value guide and redbook etc. Its a `guide` They can give you a ball park figure , unique car value guide take their prices from cars sold in their magazine and classic auction prices, Auctions can give an indication of what buyers are prepared to pay on the day.
Holdens values have risen over the years across the board but I think the prices can vary state to state , even in the newer car market cars say in Vic are cheaper than their equivilent in SA a lot of SA dealers buy cheap Vic cars with no RWC and flog them in SA as we don`t have RWC.