Here's an explanation from Glen in the Jemsite FAQ section:
"How are the JEM/UV models named. What is the "777" designation?
It is a shame that Ibanez didn't think this through better... as Ibanez let the naming get quite scattered. Except for the "77" models there can be little sense made of it.
Here's my interpretation:
777 - original JEM line series. "777" name was a no-brainer. Three models & Vai's favorite number... 777. Thought to be "maple fretboard" only but this is incorrect - now even with the new UV777BK. There is no current JEM777 so it will be interesting to see if Ibanez creates one.
77 - JEM & UV: Rear routed bodies w/ clear pickguard ala Florals & multicolors. Hopefully Ibanez won't screw this up with a front-routed 77 one day. IMHO the BSB could have been a cool rear routed 77 with clear pickguard :-)
7 - originally the "affordable" JEMs (7RB, 7PBK were lower priced than the 777 and 77FP). Now they're the high end -> 7VWH, 7VSBL & low end 7DBK.
Hindsight is always 20:20 but it would have been nice to keep 777's pyramid inlays only, 77's rear routed only and 7's vine or other inlays with front routed bodies."