Has to do with the money per year average of the contract first. Then the playing time the player receives during the regular season. So how far his team goes in the playoffs don't make any difference.
PFF grades should kind of be taken with a grain of salt. A tackle that loses a battle to an undrafted rookie is graded the same as a one that looses a battle to JJ Watt. It's like a +1 or -1 no matter who you are playing against.
Also some of the coverage grades are kind of questionable. A LB or CB or S that picks up a receiver that was not his responsibility, but no one else picked him up gets graded down for allowing the catch. But he was picking up for someone elses mistake. So should he be?
All I am saying is PFF is a good base line to grade someone but its not like its the bible or something.