Because Cutler has a sample size large enough where it's fair to bring up the averages.
You can't bring up Orton leading the league in yards, without bringing up the fact he also leads the league in attempts, or that he's 7th in yards per completion...tied with Mark friggin' Sanchez, who isn't allowed to throw anything beyond 10 yards.
Cutler is 2nd in the league in yards per completion, and 4th in yards per attempt, ahead of Orton in both categories. The one category where Orton has a huge edge on Cutler this year, yards, has got to be negated by his huge edge in attempts.
The formula for beating Denver has been the same for two years now. Let them have all of that underneath junk they throw (in place of the lack of running game), and dare Orton to beat you. To his credit, he's had some success throwing long at times this year, and is having a career year. But you also get games where he throws for 500 yards and produces 13 points.