767-300s do ORD-PHX a few times a year.. sometimes you'll see 3 flights a day to PHX using the 767-300 during it's "season"

United does what it can to predict traffic levels, which is why you'll see 757s and 767s on every route to Florida during certain times of the year, and other times you are stuck on a guppy 737-500.....

-nate