Hi Toha, I think it's because these surfaces are not totally smooth to each other:



When surfaces meet up at a sharp edge, then the fillets constructed between them do not touch each other directly. So it's building the fillets and then putting in a blended corner patch between them, that's the small piece.

It looks like the surfaces are smooth over on that side but maybe not on the other. That's one of the problems with trying to construct models by building the 3D wireframe and then filling in pieces directly to the wireframe. It's very easy for the filled in surface to not be fully smooth with each other.

Your surfaces there have control points like this:


What you would want to see for best geometry quality is something like this:



You'll get this type of structure if your base shape is an extended block and then you use 2D profile curves to trim away pieces to form the outline.

This makes a simpler "underlying surface" that has trimmed away areas. One surface made like this will be smooth throughout but if you go in and try to fill in a 3D wireframe that won't be the case.

- Michael