Can anyone explain this to me?

So I see this pop up in some of the fic communities I'm on every once in a while. Someone will post a URL to their fic along with an apology for not using the cut tag because "I'm bad at LJ cuts". What the hell is that? How can you be bad at lj cuts? There's no skill involved. Either you know how to use it, or you don't. But how can you be bad at it?