Another way to try and describe how the fillets do not "naturally" come together for this case is that the natural fillet pieces will be on opposite sides of that plane.

For example the fillet in one area wants to be like this:




But the fillet in a different area wants to be on the opposite side of the plane like this:




So those various fillet pieces kind of stay separated on opposite sides of that plane until the top or bottom spots where they would need to be connected up, that's what makes it a complex case.

- Michael