Why is there such a big difference, and is there any data about how this this reflected in later careers?
I would guess that the advice of "only do a PhD if you want to be an academic" doesn't hold as strongly over here. Does it?
(Disclaimer: I say this as someone currently debating whether or not to do a PhD, but who doesn't want to be an academic.)
I spent a semester in Ph.D. school in Denmark, and I noticed what you describe: 4 years guaranteed funding with a boot at the end.
What this means is that at the end of four years, a student has to staple all the papers they wrote, back-solve for a unifying "thesis" and then defend. No matter what.
In general, this is why Europeans Ph.D.s tend to need a postdoc, whereas American Ph.D.s can (usually) transition directly to faculty jobs.
The US Ph.D. student is encouraged to hang on and continue publishing with their advisor while they're on a hot streak and for as long as the grant funding lasts.
This allows US profs to turn the last couple years of their Ph.D. students into postdocs paid at grad student wages.
In physics, going straight from grad student to professor is extremely rare these days.
This has downsides since the student is effectively an employee of the lab. I've seen examples where conflict arose between what the lab needed from it's RA and what the student needed in order to progress with research work.
My understanding is that they are given 3 years for their PhD and do not necessarily take any courses. With a British BS being 3 years, their total time to PhD is 6 years, whereas (for physics) the average time in the US is 4+6=10 years. Not knowing anything else about two fresh PhD grads, I would take the one trained in the US over the one from UK.
I'm a British student taking my PhD in the US. The core difference is publication record: in the UK, publications while doing your PhD are simply nice to have. In the US, they are essential. This is one of the key time-stretchers (alongside having to take classes and being able to change your thesis at any time).
This roughly means that a PhD that has just graduated in the US is immediately hirable all over the world, whereas a European will not be able to take a job in the US until they've spent time building a publication portfolio after graduation.
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.)