Long story. Because of copyright issues, Lewis actually published the Chronicles twice, using two publishers, one either side of the Atlantic. The British version was published first, followed a couple months afterward by the American version. Between the publications, Lewis made minor editing changes on the manuscripts. One was to change the wolf's name - in the British Edition he was Maugrim, in the American his name had been changed to Fenris Ulf. Other changes included minor things like the location of the engraving of the Deep Magic (B: "Fire stones", A: "World Ash Tree"). The biggest difference was in Dawn Treader, where the ending of the chapter The Dark Island was significantly altered. So for years, which of these things you saw depended on which version of the book you had.
At some point, MacMillan publishers picked up the rights to the books, and got tired of maintaining two versions. In 1993, they announced that they were going to standardize on one version, and chose the British version. Thus the editing changes that Lewis had made for the American version were simply pitched, and the British version became the sole version.
Which version is "right"? Well, they're both right, because Lewis wrote them both. If you think of "the most recent version written by the author" as being "the most right", then the American version would be preferable, since usually that best reflects what the author really wanted. Since most of the changes are trivial, it doesn't matter much, though Lewis thought the changes at the end of The Dark Island were significant. I grew up on the American version, so Jadis' Chief of Police will always be Fenris Ulf to me.
Most of this information came from Paul Ford's superb Companion to Narnia, which has an entire appendix detailing the differences between the versions.