Without knowing a lot more about the game engine and exactly what the differance between the two cards is its difficult to be sure whats going on.
Things it could be are that the game is more suited,better coded to run on a dual and dosent like Quads. There could be something that the 260 is capable of doing as far as detail settings go or does differantly that means its actually doing more and so is slower.
Do you have anything like V-sync on that could cause an issue ?
Its not good to get hung up on just one game either does the result repeate itself when running other games ?