Well, its quite a long explanation, but here goes...
Ya know on film you have those age ratings with the triangles, and the circles; U PG 12 15 and 18... The old assassins creeds were 15, correct? Well, there are actually 2 age rating companies; the BBFC, which are the flim-y ones mentioned above. These guys usually rate the more adult, mature games, such as COD, Assassins creed and halo 3. The other rating company is the PEGI; these guys are a bit harsher than BBFC, and ONLY rate GAMES. Now, heres where it gets... strange.... Any game with a BBFC rating ALWAYS has a pegi rating, but its not on the box. This rating will be inevitably higher; Assassins Creed Revelations is a BBFC 15 but PEGI 18. This is just the difference between the companies opinions that seperate the ratings. Because BBFC ALWAYS rate lower than PEGI, any BBFC 18 games are ALWAYS PEGI 18 too. Another example is resident evil 4; 15 by BBFC, but 18 by PEGI.
Now for the actual reason;
BBFC have now STOPPED rating games. The previous assassins creeds were BBFC 15 and PEGI 18. HOWEVER, since only a few months ago, the less strict BBFC do not rate games any more! This means that instead of a nice 15, it has PEGI's 18 rating. It would MOST PROBABLY be a 15 if BBFC still ratedgames. But alas... they dont.