The simple answer is, of course, "Their amazing defense and a really lucky play against Cincinnati." But consider the following two quarterbacks, each with the same number of pass attempts:
Quarterback A: 4,526 passing yards, 25 TDs, 18 Interceptions, 86.0 passer rating, 6.8 adjusted yards per attempt
Quarterback B: 3,937 passing yards, 24 TDs, 16 Interceptions, 79.6 passer rating, 6.0 adjusted yards per attempt
Which one is better? Quarterback A, but not by a large amount.
Now, suppose my team has quarterback B. I'll trade him to you for quarterback A. Not a good deal for you, but, depending on circumstances, maybe one you would make...
Oh, and I'll also throw in two first-round draft picks and a third-rounder. Can you toss me a fifth-rounder, maybe, just to even things out a bit?
I bet you're taking that deal.
Quarterback A's stat line belongs to Jay Cutler in 2008. B's stat line is Kyle Orton's stat line in 2008, adjusted to have the same number of attempts as Cutler. The reason Cutler's numbers looked better in 2008 was solely because of his high number of attempts. Plug Orton in for another 200-odd attempts in 2008, and his numbers start to look like Cutler's.
When the deal was made, I was skeptical of both sides. The conventional wisdom of Cutler as a franchise quarterback still lingered in my brain, despite my certainty that his "big numbers" were more the result of his number of pass attempts. Orton, meanwhile, while not great, was reasonably efficient in 2008, few people could dispute that he enjoyed a better receiving corps (Brandon Marshall and Eddie Royal) in Denver than Orton did (Devin Hester and Greg Olson/Matt Forte) in Chicago.
(The Vikings also were supposedly interested in Cutler, and he probably would have been an upgrade over Tarvaris Jackson and Sage Rosenfels, but I was hoping we wouldn't give up the house to acquire him.)
Admittedly, we're only looking at one season's worth of stats here, but that's about all we can do. Orton was awful in his rookie year, starting for the Bears in place of the injured Rex Grossman, and played sparingly in his second year. For Cutler, one year looks pretty much like the other. I'm too lazy to compute all sorts of advanced stats, but his passer rating his first four years in the league (counting this one) are 88.5, 88.1, 86.0, and 86.9. Decent, but not something I'd want to give up three high draft picks and a reasonable quarterback for.
So far, Cutler's performed reasonably well (the opener in Green Bay aside), but he's still looking like about the same passer he was in Denver -- willing to put it up all the time, but interception-prone. Meanwhile, we do harp on Denver's defense, and it's amazing, but Orton has nine touchdowns versus just one interception and a passer rating over 100. I don't think he's that good, but far too many people were just thinking of him as subpar, if not outright bad, going into this season.
But hey, look at how much Cutler is helping the Bears' running game. Matt Forte's 3.4 yards per carry is clearly the result of improved quarterback play.
So the next time you hear someone say Denver's just having a good season because of their defense, know that that's just part of the story. For years, it seemed like the Broncos could make any running back into a 1,000-yard back. Nowadays, maybe they can make any quarterback into a Pro Bowler...