http://www.starwars.com/movies/saga/20080826news.html
^^^^
Star wars fools decided to split them into trilogys
WIKIPEDIA:
A trilogy is a set of three works of art that are connected, and that can be seen either as a single work or as three individual works. They are most commonly found in literature, film, or video games, less commonly in visual art or musical works...
Adding works to an existing trilogy-
Creators of trilogies may later add more works. In such a case, the original three works may or may not keep the title "trilogy".
Richard Wagner's epic series of operas, Der Ring des Nibelungen, is sometimes referred to as a trilogy even though it consists of four works: Das Rheingold, Die Walküre, Siegfried, and Götterdämmerung. The first work, Das Rheingold, is more correctly considered a prelude despite the fact that it is longer than most operas. Performances of The Ring are usually billed as three nights plus a prelude...
The six Star Wars films are generally separated into two trilogies; the "original trilogy" (the three films released between 1977 and 1983) and the "prequel trilogy" (the three films released between 1999 and 2005, which take place before the original three films).