Why do some double negatives oppose with the natural or instinctive meaning which apparently occurs from the context of sentence they're applied in? [closed]

There would be more double-negatives like these, but I specifically want to mention "This couldn't be further/farther from the truth".

When this phrase is used, it apparently incites the sense that "This couldn't be a lie, but much nearer to the truth". It isn't apparently recognizable as double-negative when someone(even with much familiarity with English speaking/listening) hears this for the first time, but innocently could interpret just the opposite that is mentioned above in quotes.

If the speaker had just used an additional term anymore/any/more in this then it could have made sense and become recognizable as double-negative "This couldn't be anymore further/father than truth". And to support this belief, let's compare this with "I couldn't care less", now when someone hears this phrase, it almost becomes clear that speaker shows "uncaring" attitude towards something/someone.

What could be the reason that these kinda "dubious" double-negatives came to be and widely used despite their inherent nature of inspiring "reverse" than what is intended meaning by the speaker ?

The phrase, couldn’t be farther from the truth, isn’t a double negative. It does mean what it says - that something is not true, and is, in fact, very much unlike the truth.