It's likely just nomenclature differences...Apple did at one point call the cards in iMacs from 2007-2010 by their desktop equivalent names even though they were still MXM boards and not the same GPUs found in desktops at the time. Likely it could be done that way here to either make the 680MX seem more desirable, or to make the 675MX seem more respectable. Either way, it's Apple and/or NVIDIA trying to pretty something up.