You hear people say "he doesn't know any better" or "she doesn't know any better" as if that excuses the bad behavior. Does it?

We also hear "ignorance is no excuse". So which is it? "When we know better we do better" per Maya Angelou. Well maybe in her world but not my world. Of course what is "better"? Is there any standard we can apply to measure it or compare it? Is "better" always subjective and personal?