They're not the same, nor do they mean the same. To essentially paraphrase the video of David Mitchell, if you put caring on a scale then saying you couldn't care less puts you at a 0 level of caring. You could care more, but you can't care less. If you say you could care less then all that tells us is that you're not at a caring level of 0 which is what the use of the phrase is meant to imply. Saying you could care less signifies that you could care a lot, you could care a little, you could care a moderate amount. Its meaningless as a phrase to gauge how much you could not care which is why people are saying (or attempting to say) the phrase in the first place.