Um, are you asking why they both have inlays, when now-a-days only 'DX' or 'EX' or some other thing are the only guitars with the extras?
All 7xx guitars from that period(because this doesn't hold true to the newer Japamese 7xx) have binding and inlays.. The DX designation was just for the extras on top of that, ie color-matching..
up to 93(According to catalogs for the US)
RG550 -> RG750
RG560 -> RG760
RG570 -> RG770 (only w/direct
mount pickups)
RG770 -> RG770DX
Of course, there were no RG5xxs with binding and inlays during this period that I know of.. Maybe in Europe, lol.. After they ditched the 7xx, they started having 5xxs with binding/inlay and giving them a designation(LTD, EX, DX, etc)..
Personally, I wish they'd bring the RG7xx back to the US.. I'd rather have binding over flame tops, all the Prestige models should come with it for the price..