I overheard asenior scientist say that scientists in US/Canada make themselves known in the science world during their PhD and in UK you tend to make a name for yourself during a Post doc.
US/Canada PhD courses can take longer due to the amount of teaching many of the students have to do as part fo their TA funding. So US/Canada PhD students can be more attractive for academic posts because they have, onaverage, far more (and far better!) teaching experience.
In UK you can concentrate on your research and not worry so much about courses and teaching. So you can still be comparably as productive as your US/Canada counterparts during a PhD.
In the research tables the US dominate, maybe they do produce more productive researchers?