I know the answer to this question. I was a research astronomer for 10 years. During that time I knew many people, from various universities, who said that during their PhD their supervisor would "demand" that their (the supervisor's) name be put first on all, or some, of the research paper, even though the supervisor had not significantly contributed to the paper.
Since professors of departments are more likely to have more PhD students than their junior colleagues, they consequently end up with the greater publication rate.
I am glad to say that I did my PhD at Glasgow University where this despicable practice did not occur.
Dr GEOFFREY K FOX
The Bungalow Rydal Street Gateshead