By the end of undergraduate they will have very much cached up (American schools may be bad, but Unis are top notch) but then in places like UK, students would have been a lot more focused. So once you move to PhD the first couple of years in the USA are more similar to what in the UK we call a master. Hence only the last few years of the PhD correspond to the 3 years of the European phd.
Overall I think the two system are equivalent in duration, it's just that in the USA they start from a lower step of the ladder.
(on top of this there may be difference in the focus: e.g. papers vs. thesis, but I am only talking about the durations.)