most probably for the same reason that one car runs on trionic 5 and needs the red unit, while the other one runs on trionic 7, which requires the black one to run properly.
actually had a red one fitted on trionic 7 saab 9-5 by some guys who were fixing the car while on being on road and it was running well till guys at saab service noticed it and had it switched. the red one runs fine now on my 9000 saab.
Quite possibly, since Trionic 7 is a lot more advanced than Trionic 5. The question is, would you get any benefits from plugging a T7 DI into a T5 car? Unlikely, as the T5 would not be able to support any additional features. In fact, it may not even work - last I remember, it was a source of heated debate on the forums whether a black DI would work correctly in a T5 car.
ah yes - but if your getting Motec to write a config and prepare a harness for a DI cassette do you ask them to work with a red T5 or a black T7? My guess would be the T7 - but I don't know what the differences are.