maybe its the blood an gore, plus people tend to use the ratings as a advertising ploy as well, the idea is kids will want the game more if they think it`s forbidden fruit, and the older users expect more hardcore gameplay, plus have you got far enough to find those rocket women?, from the demo I seem to recall they have all the old QuakeII baddies for the early levels brought up to date, so maybe the rocketlauncher girls show up later in the game and are scantily clad or something like that.
Windows: 32 bit extension/graphical shell for a 16bit patch to an 8bit OS originally coded for a 4bit CPU, written by a 2bit company that can't stand 1bit of competition, now available in 64bits.