Fuel Economy and diff ratio

[Frosty]

Far as I know Frosty all late 60's thru 70's auto and 4 speed manual Holden stuff is 1:1 top gear (not sure about powerglide and early Hydramatrics though).

I was thinking that ther might be some slip in the auto? Why would the diff on the auto be 2.78 and a manual in the same model be 3.36?