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.
From an ownership persepctive the Parkard Merlins are better to own since they were mass produced using modern manufacturing techniques and therefore parts e.g. pistons, crankshafts etc will fit from one angine to another. The Rolls Royce engines were hand finished with each part being finished by hand to fit into the journals etc and therefore are not likely to fit another engine.