Department A has a lot of faculty doing exciting research in areas you're interested in, but those professors' CVs generally show little evidence of co-authorship with students and say nothing about advising or dissertation committees.
Department B has a smaller faculty in your subfield, their interests aren't perfectly aligned with your own, but they're putting out a lot of papers with grad students.
How do you weight these factors? Throughout the application process, "best fit" was supposed to be rule number one. But I acknowledge that my interests are malleable and that a perfect fit doesn't mean much if your advisors don't give you the time of day.