It's just because different people wanted to add to the story or change it up a bit. If your looking for the original it's "Morte d'Arthur" by Sir Thomas Mallory. It's a pretty big book, though.

that is not the first version...the first complete story was within the history of the kings of britain!!

Some time ago, I read the "The Crystal Cave," by Mary Stewart. This was the first of a well-written fantasy series about Arthur, using the Dark Ages of Britain as its backdrop; not the Age of Chivalry, which had not occurred during his watch. Archaeologists think that he was a Dux Bellorum (War Leader) in the 5th or 6th Century. They attribute a tomb that they found to him. Stories about him were passed down verbally until a written language was developed, centuries after the fact. I think that Beowulf (the first "English" writing) was finally set down in the 9th Century. A few hundred years later, Geoffrey of Monmouth write his Historia Regnum Britanniae (History of the Kings of Britain) where Brutus (of Caesar Fame) traveled to Great Britain and became its first king...and a whole lot of other legend and fantasy, true and false. Arthur reached for Excalibur in this book. There's so little known of this Dux Bellorum that an author can practically make up what he/she likes. And that became a large branch of the Literary Industry in Great Britain. Here's a well-researched article: http://en.wikipedia.org/wiki/King_arthur

Why are there so many contradictory versions of Greek myths? Why are there so many contradictory versions of the story of Siegrfried/Sigurð and the fall of the Nibelungs? Why are there so many contradictory version of the story of Superman? Why do film makers so often produce films with great differences from their claimed sources? Why do remade films often differ greatly from the original. Why does Shakespeare’s “Hamlet” not more closely resemble the “Amleth” of Saxo Grammaticus? Sometimes it is because a story teller when retelling a story may not fully remember all the details, and may fill in for lack for memory by invention or by adapting material from another tale. Sometimes a story tell may purposely change a tale to supposedly improve it. Sometimes the tale may be all or mostly invention. If the audience wants a new story of Lancelot, a teller may give it to them, either mostly from his own head, or by changing a tale earlier told of another hero. Accordingly in Arthurian legends, Gawain, Caradoc, and Lancelot are each the subject of the tale of a beheading contest. Perceval achieves the grail in most versions, but Gawain achieves it in one version, and Galahad aided by Perceval and Bors in another. Here this partly comes about because Chrétien de Troyes lest his account of Perceval and the Grail incomplete, and various authors wrote inconsistant sequels. So Heinrich von dem Türlin in his “Diu Krone”, a work devoted to Gawain, naturally made Gawain the grail hero. Most authors went with Perceval, as was probably Chrétien’s intention. The authors of the Lancelot cycle could not make Lancelot a grail hero because of his adulterous afair with Guenevere, so they introduced the son of Lancelot, Galahad, kept Perceval as a secondary figure, but also introduced Bors, Lancelot’s couson as a third figure. And by having Lancelot temporarily repent his relation with the Queen, they could also ascribe grail adventures to Lancelot. This leaves Gawain to take on the role of the unregenerate sinner, very different from his role in earlier grail stories. Geoffrey of Monmouth in his “Historia Regem Britannia” refers to Arthur having defeated a giant name Ritho who wore a coat made of the beards of kings he had defeated. See http://www.lib.rochester.edu/CAMELOT/geofhkb.htm . Later references refer to Arthur’s battle with King Rion fairly often. Note the change of name in French. If the place name Lothian becomes in French Loenois, then Ritho can become Rion. But the only full account of Arthur’s slaying King Rion that has come down to us occurs in the so-called “Vulgate Merlin”. See http://www.lib.rochester.edu/CAMELOT/geofhkb.htm . However the author has obviously adapted this to his setting. King Ban and King Bors, for example are present because they are important characters at the beginning of the story of Lancelot, to which the “Vulgate Merlin” is designed as a sequel. However in the verse romance “Li Chevalier as deus epées”, the giant with the coat of beards is named King Ris, and he is slain by the hero, not by Arthur. The hero’s name is here Meriadeuc, though he is mostly just called the knight of the two swords. Of course, if questioned, the author might have claimed that his King Ris was a separated bearded king from King Rion. The very beginning of this romance is on the web at http://www.yorku.ca/inpar/Knight_Two_Swords.pdf . However in a work sometimes called the “Post-Vulgate Merlin” the, knight of the two swords is introduced at the beginning of the war with King Rion. Here the knight is named Balaain, and he has a brother named Balaan but otherwise events follow the verse romance very closely. Balaain and Balaan defeat King Rion with Merlin’s aid. But the author apparently knows the version in which Arthur is the one who defeated Rion, and now introduces Rion’s brother Nero, who takes over Rion’s army and claims that Rion is with the host, but at the back. Accordingly we now have an explanation as to why two versions exist, that is Arthur actually fought Nero, not Rion. See http://members.terracom.net/~dorothea/baladro/Chapter22.html and the three chapters following. This later version is the one that appears in Malory’s “Le Morte d’Arthur”. See http://www.sacred-texts.com/neu/mart/book01.htm . Malory’s account is one of the latest medieval Arthurian works, being written over 400 years later than Geoffrey of Monmouth’s account.