You got to remember that in Don Revie's Leeds's days, there was only Yugoslavia and USSR. Now we have "gimmes" like Moldova, Kazakhstan, Macedonia, Lithuania, Montenegro, Georgia, Estonia, etc. Sniffer's stats can't be accurately compared to Rooney's, even if they're comparable.
Also we played more games against Wales, Scotland and N. Ireland back in the 70s, and they were all better than they are now.