Godef has it right. This was discussed hotly when it happened iwth the realignment. While it's not "geographically correct", it did the best job of preserving the hotter rivalries between the various teams. The Dolphins had long histories with the other 3 teams in the AFC North, and the Colts did not. The Ravens (nee Browns), Browns, Steelers are obviously a foursome with a long, bloody history of intra-divisional hatred, and the Bengals fit in better with that group than the Colts. So the Colts got slotted in the next best place. The NFC equivalent is Dallas, in the NFC East, while many other teams would make more geographic sense (Carolina Panthers for one). The Seahawks also moved from the AFC to the NFC just to make it all work better (16 teams per conference).