GranTurismo - reason for two exhaust pipes or four?

[bass2rez]

Hopefully a quick question that I've not found the answer to.. what is the reason that some GranTurismo's have two exhaust pipes and some have four exhaust pipes?

Thanks,
Steve

[bass2rez]

Many thanks for the replies. Seems the answer is not as straightforward as I expected.

Thanks,
Steve