Giving up 600 points is better than 500 points?
It's possible that it could be depending on how the points were scored. If the defense gives up 500 points on drives that are really long on average and thus keeps the offense from having many chances to score, it could be worse than a defense that gives up 600 points in a different fashion.
Flip it around and think about it offensively:
Say the Packers offense has a game in which they have 14 legitimate possessions (i.e. they get the ball with at least 1:00 minute on the clock) and they score four touchdowns-- 28 points.
And then say in a different game, the drives are all longer so the Packers only end up with 8 legitimate possessions and they score three touchdowns-- 21 points.
In which game did the offense perform better? In the one where they scored on 29% of their opportunities or in the one where they scored on 38% of their opportunities? Or to put it another way, in the way where they scored 2.0 points per possession or the one where they scored 2.6?