The fact that he's thrown for over 5000 yards in a season?
Stafford has averaged more passing yards per game over his career than Manning, Rodgers, Brady, or Brees. It's because he's averaged 40 attempts per game over his career, while they've only averaged 34. There are several reasons for that difference, but here is the main one:
2009-14 Pass Rating, 1st-3rd Qtr
Manning 110.5
Rodgers 109.4
Brees 103.7
Brady 101.0
Stafford 85.3
While those other QB dominate in the first three quarters, Stafford is just average. While their teams are running the ball and protecting the leads they've built thanks to their 100+ ratings in quarters 1-3, Stafford keeps throwing. In the first half of games, Stafford ranks 10th in attempts. In the 4th quarter, he moves up to 3rd. All the other QB I mentioned move down (on average, from 4th to 9th). While Stafford continues to sling it and pile up yards, his career 4th-quarter passer rating is 79.6.
The NFL hasn't used passing yards to determine its passing leader since 1937.