The 970m has a 192 bit memory bus, the 980m has a 256 bit memory bus.
Incidentally the 970m's bandwidth is proportionally less (3/4 that of a 980m).
Which implies that it's addressing 3/4 the amount of RAM chips compared to a 980m. So a 6GB one will be using memory chips at double the density, which again means it's not an issue.
Really, they should have just arranged it like that for the desktop 970.