Which is why the rankings, especially area rankings, should always be taken with a shaker of salt.
My favourite example of this is comparing the phil of religion PGR rankings from 2011 to 2015. In 2011, UNC-Chapel Hill was ranked 2nd (I think) in the English-speaking world for philosophy of religion. In the most recent ranking, it's nowhere to be found. Why? Because the Adams family moved from UNC to a 3-year position at Rutgers' new Centre for the Philosophy of Religion, which is also why (along with Zimmerman) Rutgers is currently in the PGR's second tier.
Especially when dealing with smaller/less well-known AOIs, the PGR should only be used to point you towards those departments where big names in the field are to be found... and even then, if you've done your homework, you should be able to do that without the PGR's help :)