This is just a kind of port of Fable, from the xbox. It was given the additional title of Lost Chapters because it has additional content, and many improvements. Which is why it was also re-released on the xbox. So I understand how it can be confusing...
Actually, Fable was originally designed on a pc and then ported to the XBOX. While Lionhead was creating the port they were forced to cut content, i.e. quests, items, and much of the storyline in able to make the release date Microsoft gave them. The Lost Chapters for the PC is the "original" pc version first created and the XBOX Lost Chapters version is the original port with all the cut content added back in. It is not an expansion pack in the true sense but the actual original, full-length game Lionhead created and hoped to release. If you own the PC version you are in for a real treat. If you have a nice graphics card capable of viewing high resolution with , anti-aliasing and the newest shaders, you'll be able to view the game in such quality and beauty that it'll amaze you. Lionhead states in one of their PR articles that the PC version boasts over twice as many pixels as the XBOX version can handle. I believe them, it is truly a beautifully rendered game for the PC.