I suggest it's not paid links, but run of site links (frequently paid) that are being addressed. Further, it's possible that a ROS link from a "PR9" site gives 9 "votes" while a ROS from a "PR4" provides only 4 "votes." I'm suggesting ROS links may be devalued on a scale, based on the quality of the site providing the ROS.
Disclaimer: I have no evidence, it simply seems like a reasonable thing for Google to do (to me at least!) all things considered.