How do you know if you are "good" at something?

People usually base their claims only on their own personal experiences. As you stated, if a student does well in one class, but not in another, then he/she will claim that they are good at that subject. Obviously, they are not comparing themselves with those with PhD's in the field. When one says I'm "good" at something, they're only claiming that they're better at that subject, than the other subjects they have been exposed to.