And when you search one airline it shows up with ALL that airline's flights?
Well, I doubt that theres an airline that has 20 flights between two cities.
In your above example, searching each airline separately, it has them all.15 on CO, 14 on UA and 6 on US.
As KVS said earlier, the issue is displaying more then 20 flights. Once there are less then that amount in the particular search, it will show all flights.
BTW, do you want me to search a flight for you?