I seem to recall when the first book came out, that it was planned as a trilogy. After the second book, it was going to be a double trilogy. I don't think Jordan's overall plan reached his final 12-book estimate until book 8 or 9, and Sanderson couldn't even fit the final material he picked up into two books, so it hit 13 by the end.
Now, there's nothing wrong with that in general. Sometimes you write an outline, and in the process of penning down a quarter of the story and fleshing out the characters and setting you come up with more and more ideas. There's two ways out of that: either you let the story get control of the writing, resulting in a historical-feeling narrative with no clear overall plot, or you take the time to edit and revise what you have into smaller, self-contained stories.
I vastly prefer the latter, especially if there _are_ hints to an overarching plot. 13-book meanderings like Wheel of Time don't do it for me these days, but Dresden Files (15 books and ~20 short stories) or the Honor Harrington series (13 books, plus 18 spinoff short stories or novels)? I will chain-read those like crack, and still feel satisfied almost regardless of where they stop.