It definitely doesn't make sense, but I think it's probably a result of a few folks eliding the "n't" on "couldn't" when speaking, then the phrase getting interpreted by the listener as a single phonic unit "could care less", which carries the semantic definition "couldn't care less".

It's just that the listener's brain understands the speaker based on the context without needing to deconstruct the clause and notice the elision, and so the gestalt phrase is what's remembered, and, later, recalled.