I have noticed this too, I've been assuming it has something to do with the horizontal drop-in style bobbin letting it sort of store up in there, vs. the old vertical front-loading bobbin style where the lint drops out on its own easier. I do remember occasionally finding lint falling out of my old machine when I'd open that little door to change a bobbin. But I really don't know! I've always sewn with cotton so that's no different.