32bits is the maximum for the last 512MB, because of the way they have disabled the ROP/L2 (which normally would make them also disable that MC) to which it should be connected and it operates separately from the other MCs, so the maximum bandwidth is just for one MC (32bits) while reading/writing to that, while the other 3.5GB can use the full 224 bits from the 7 MCs combined (and the 980 can use the full 8 MCs = 256bits)
I can see why you think (effective) bus width is between 224 and 256 bits. But each 32 bit MC is connected "only" to 512 MB memory chip. The problem with 970 is that read or write cannot be performed by all eight of them at once. They can still claim 256 bit bus performance if they ensure last partition will do the opposite. Whether it is possible I cannot imagine. But if they can handle it, then it also implies all the memory chips will be in use at once.. aren't 4 GB still possible?