Why European amphibious veterans still needed Pacific retraining

How different was the amphibious training of US Army, British, Canadian, and US Marine Corps formations?

Different enough that European-theatre formations were expected to retrain in Pacific doctrine before taking part in an invasion of Japan.

Amphibious proficiency did not transfer unchanged between coastlines and enemies. The Pacific system differed sufficiently from European practice that forces moving east for an invasion of Japan would have learned a new doctrine. Experience helped, but terrain, defences, fleet support, and landing methods still changed the problem.