I could care less

Why do some people online write “I could care less” when seemingly meaning “I really don’t care” or, in fact, “I couldn’t care less” – the opposite of their written meaning? Is this some kind of “bad means good” or “sick means great” meaning inversion, or are the users oblivious to the fact that they’re saying the opposite of what they mean? I guess by writing this I’ve proved that literally I could care less. D’oh!