Hi Steve - that probably means that the solid is confused about which part of the volume is the outside and which part is the inside, it's ending up reversed from what you would usually expect for that shape. Any reversal in the inside/outside region will make things like boolean difference behave like union instead.

The main way for that to happen is if you've got any self intersecting surfaces that sort of squiggle back on top of themselves. It looks like some of your side surfaces have that problem:










That surface kind of hooks back over top of itself and so it is actually folded very tightly back on top of itself in that area there, that's what is likely making for a problem with the solid's structure.

Try to examine the curves that were used to generate that piece and see if the curves used had a little hook type thing at their end. You will probably have to pull some points around (and then undo) to really see it, it can be hard to see things that fold back over themselves with a sudden 180 degree turn like that.

- Michael