Airlines do some crazy shit with their pricing.
First off Virgin Atlantic and Delta work together so I went to their websites to find a flight from Pittsburgh to London. Knowing that this is not a direct flight and the connection is New York I found that two tickets to go from Pit-Lon on either carrier would cost roughly $2500.
With that being said. I decided to look further into this and broke the flight down into two parts and priced them separately. The first Leg from Pittsburgh to New York on Delta is about $600 for two people and the second one from New York to London on Virgin Atlantic is about $1200. For a total of $1800.
1800 < 2500
Same flights, Same planes, Different price... O the wonders of the travel...