What is the difference between someone of an IQ of, say, 81 and someone clocking in at only 79? Aside from margin of error making them completely interchangeable, one might qualify to be defined as retarded, while the other, just a jackass.
What does it mean to be judgmental? We might make common sense judgments about either individual, we may even perceive both as needing help, but one, we might cut a whole lot less slack than the other. Easily slipping into judgments that are distinctly moral in character, as if by being over the entirely arbitrary line of 80, they're choosing to be stupid, while the other is not.
People make all sorts of judgments and exemptions like this towards each other, based on these categories we put people into. When a person's existence can be summed up into these neat labels, that exempts them from our normal measures of free will and self-determination, to some degree. Their failings can look more understandable, because they have these authoritative definitions slapped on them.
These distinctions are illusory social constructs. On some level, intended to be practical, we seem to have this coding loosely effective for discerning who we should help by lending a hand, and who might better be helped by shaming into helping themselves. When teaching someone to swim, it can be better not to physically throw a person into the water, if they can be pressured into throwing themselves. This tendency manifests in all sorts of ways, from parenting techniques to theories of crime deterrence inherent to our legal systems.
Sometimes that may be an effective thing to do, but there's often this tangling of good judgement and being judgmental. A tendency to feel that this is the righteous thing to do. Unless we have these distinctions which exempt people from such treatment. You don't throw someone into the water who's clearly incapable of learning to swim, that would be psychopathic, but what of people who prove unable to swim for less obvious reasons? Do we protect ourselves from sympathy and questioning our own good judgment, by blaming them for drowning themselves?
Reality is a whole lot messier than the categories we put people in make it out to be. As far as I can tell, the world is full of people who seem to insist on adhering to their failings, despite all burdens of social judgment or physical upheaval. As if with or without these labels, we are who we are. Struggling to be who we are, within the circumstances of our lives, to the best of our abilities to understand and utilize what we can.
Of course that can be incredibly frustrating, all too often screaming out to be some other way, but such is life. When having faith in some sort of metaphysical ability to transcend such limitations means being callous and impractical, I think that might be the key difference between good judgment and being judgmental.