The bus width is irrelevant. Its the total bandwidth that is relevant to the performance - This is affected by not just bus width, but also vram speed and also any compression processes.
But that's exactly what they state, total bandwidth.
GTX 680 - 192GBs.
GTX 770 / 970 / 980 - 224GBs.
If the GTX 680 was already hitting the ceiling, I am not so sure about the others, even with compression technology and all. And the GTX 670 (with the same bandwidth of the GTX 680) was having trouble to follow the HD 7870 in 8XAA. They are just too close to say it is safe. Hence why I think a review could shed some light on it.
as i can understand different GPU will have different efficiency in using their bandwidth. despite having much less bandwidth 680 still perform much better than 7970 in general (which means kepler is more efficient than GCN in term of utilizing available bandwidth) . so in the test you link above 7870 is beating 670 when 8x MSAA being use. so did you think bandwidth limitation causing such problem. then again look again at 7870 spec.
it has the same bus with 670 which is 256bit. 670 have total bandwidth of 192GB/s while 7870 only have around 154 GB/s. it has the same memory width and much lower bandwidth than 670 then why 7870 slightly beat 670 when using 8x MSAA? the way i see it AMD GCN architecture are much better in handling MSAA than Kepler.
but there are also talks about 680/670 being bottlenecked by bandwidth. but to simply attribute that 680/670 being weak with higher level of AA because lack of bandwidth is not entirely accurate. it is more like 680/670 in general being bottleneck by it's total bandwidth.
You need to see the whole picture. Its easy to just point the finger at "memory bus width" without understanding how it all works.
Nvidia wouldnt intentionally go with something that would make its product perform worse - There is obviously other benefits to using a smaller bus in some cases.