328i is a different DME part number than the 325i/330i. Might have something to to with the internal circuitry.. I'm not sure, but it's a good guess.
Also, all 3/2007+ bmw's use a different interface protocol (D-Can vs. K-Can), so there is probably some different things AA needs to do to program them. But obviously, there are 328i's made before that, so I don't know if this is much of an issue.