I think it's to do with the heritage of each engine. While both were V12s built by Rolls Royce I don't think they have the same heritage. One of the two from memory can trace it's heritage back to a Curtis Wright V12, probably the Merlin since it turns the same way as other American types.
As I understand it, the USA took part in the Schneider Trophy races in the early twenties - very successfully - with a seaplane powered by a Curtiss D12 'Wetsleeve Monoblock' V12 engine. The Air Ministry were very impressed by these engines and bought two of them for RR at Derby to look at.
In 1931 the S6B won the Trophy outright with an RR 'R' type 36 litre, supercharged engine, using much of the technology from the Curtiss D12. Very few years later and developed directly from the 'R' type, although shrunk down to 27 litres, the
Merlin was produced. Of course it was supercharged, the major difference was aneroid control of the supercharger gearbox and boost.
As development went on and the
Merlin got more and more powerful, it became obvious that there would be a limit on just how much. I believe this was the reason why RR went back to the original 36 litres, even though there was, I believe, another stage to the supercharger and another gear on an otherwise very similar engine? The difference was enough to call the engine
Griffon, although it is difficult to see why RR decided to make the engine turn in the opposite direction?
Roger.