About 2 years ago, when DL scheduled hourly flights to DCA, LGA, BOS, these flights departed primarily from the T-Gates. With the delivery of the 767-400, there were only a few gates in ATl where these planes could park, and now the T-gates and A-South (A1-18) are home to the 767-400's.
There is some logic as to where scheduled flights operate from. The DCA flights require a quaranitnes environment for security screening, so they now operate out of E-Concourse. EWR has gone to B-Con. The MCO flights primarily depart from A-South. Most of the West coast flights depart from A.
Most of the rivalry from years past has faded. A-Concourse in ATL used to transport more people in 1 week than LAX would do in a month, and what DFW would do in 1/2 a month. It was affectionately called Atlanta's volume leader.