Hi Bdaniel, or another way to put it is filleting means "build a rounded surface made up of arcs of this specified arc radius along these edges".

The way that is specified (being defined by an "arc radius value"), makes stuff like this work, like if you have 2 surfaces meeting up like these lines here (imagine these are surfaces as seen from the side):



Then you can apply a fillet of radius = 5 between them, which will build a connector arc of radius 5 that is tangent to both lines, like so:



But now imagine that instead of having 2 lines that meet at a sharp corner we instead have 2 curves that are already smooth where they meet like this (here I've selected one of the 2):



In this situation there does not exist any arc of radius = 5 that can be placed in between those 2 pieces, basically the mechanism of how fillets work is for applying the arc rounding onto sharp features and the way that they are specified just does not apply to already-smooth-meeting areas.

For smooth meeting areas you can instead cut back some space between them and use Blend, it's a different construction method.

- Michael