It's because they grade play after play and don't grade impact of individual plays. There's little value to it on game impact.
So Rousseau having 3 sacks with huge impact may not offset other plays he technically lost his matchup. Shaq has some real bad plays but also makes a lot of volume. And as someone else stated, some are clear wins/losses, pressures, tackles, etc while others are subjective (like coverage). A guy may not be thrown at but that doesn't mean he's in good position. On the flipside a guy may be in a WR's hip but the pass is completed anyway. Then there's zone coverage, who's responsible, etc.
I wasn't able to watch this game so i have zero imput on how guys faired. Just an opinion of what i see from sites like PFF.