Stats for European schools can also be different. I know of some that have short PhD programs but require you complete a masters before applying.

I took 7 years from the start of grad school to my PhD. The main reason was that my first thesis project was an utter failure.

My impression from talking to a lot of people, is that the European schools simply manage the PhD process to make sure it doesn't drag out too long. It's expected to be shorter, so it's shorter.

My best friend and I both did PhDs in astronomy starting at the same time, me in the UK, him in the US. I completed in 4 years, he took 9 years.

The big difference was that the PhD in the UK launched you straight into research on day 1. No teaching, no courses. He spent the first couple of years in courses, and a lot of time throughout the degree in teaching. In some ways I envied his coursework, since I was basically on my own from the start (his grad courses sounded very thorough and interesting). However I do not envy the 5 extra years it took him.

That was all a couple of decades ago now, so things may have changed.

Indeed, my graduate coursework was full time for 2 years, and the failed thesis project probably cost me 3 years.