Well the low-level implementations differ from cpu-to-cpu. Even between Itanium releases the width changes. They don't want to get bound to a single implementat

Well the low-level implementations differ from cpu-to-cpu. Even between Itanium releases the width changes. They don't want to get bound to a single implementation as techniques improve and new approaches are found. Otherwise, each time they discover something new they'd have to release a new ISA and backwards compatibility was the most commercially influential aspect of a chip. Now, it used to be that super-computer were made with custom chips, with custom ISAs, but the advantage of mass consumer parts is too high. So in general the decoupling of implementation and programming language allows for enough wiggle room to allow independent evolution without being rigidly coupled.