Hi Warren, do you mean you're trying to fillet just that one single edge all by itself instead of the whole sequence of them?

It's more surprising to me that the inner one actually works - it generally should not work to fillet just one single edge of a sequence like that because you're asking the filleter to kind of open up a hole in your model and it doesn't know how to fill in that hole. Check out here for a related example:
http://moi3d.com/forum/index.php?webtag=MOI&msg=5091.4

In some situations it can fill things in by extending the surrounding surfaces, but that's a complex operation and it's difficult for that to work very well when the surrounding surfaces are meeting at a shallow angle like you've got here.

It's much better for the filleter to have surfaces that either meet each other smoothly or come to a more distinct sharper crease where they meet. Here the different arc segments that you have making up the boundary are not smooth with one another, they form a crease where they touch but it's at a shallow 15 degree angle, so somewhat close to being smooth but not actually smooth. These kinds of situations where things kind of approach being smooth with one another make things more difficult for the filleter because when it tries to extend pieces and then intersect the extensions, the extensions kind have a region of overlap with one another rather than making a crisp intersection.

Normally for a shape like you've got here you would want the pieces to be smooth to one another and then when you were filleting it you would want to select the whole ring of edges around the entire object to fillet rather than just one single edge.

- Michael