Can someone please explain how "I could care less" and "I couldn't care less" can possibly mean the same thing?

I have always interpreted "I could care less" as an ironic statement, meaning "In fact, I couldn't care less."

I imagine it spoken in the same tone of voice as "yeah, right", in which the person is not really agreeing that you are right at all. It's a statement which is meant to be interpreted as its "opposite."