Could care less is nonsense..who actually says "I could care less" in real life unless you want to look like a dumb ass....the term in full is "could not care less" meaning, the amount he cares about retina is so small he could not care less unless he cared zero! Anyway, to be American about it "whatever"
For example:
Currently I'm thinking about the Mini but think it might not be pocketable enough. So I care about it.
If I buy it and decide it's not pocketable enough then it is out of the running and I won't care about it at all.
Therefore I currently can care less about the Mini than I currently do which makes it correct to say I could care less about it. Saying I could not care less is incorrect at this time because it implies that its impossible for me to care less about the Mini than I currently do, which as I've shown is incorrect.
Now if I later decide I don't want the Mini, it would be correct to say I couldn't care less about it because I then wouldn't care about it at all. Which is not true currently.
Don't get upset about your #GrammarNaziFail.