Meaning of "not very good"

Why does not “not very good” almost always mean “bad”? You’d think it should used as in “It was good, but not very good.”

IHMO because I doubt there’s a factual answer.

I’ve always considered the expression as something used when the speaker is attempting to be charitable in pronouncing the judgement. As such, it is euphemistic in nature, and like all euphemisms doesn’t go far before sounding strained and dishonest.

Or it could really mean great. Great is not very good because it is better than very good.

The phrase starting with a negative has a lot to do with its negative connotation, I think. It’s usually fruitless to question the logic of idioms too closely, though.

Double plus ungood.