My guess would be scaling. Off the top of my head and a brief Google search seems to lead to 680m vs 780m using slower 4800mhz and 5600mhz memory.

Which leads me to now think they both scale pretty equally at this RAM transfer speed. However at 6400mhz, the RDNA 3 780M could probably start scaling higher compared to the 680m. Guess we’ll never truly know.