If I score 70% in an exam, am I "tending to ace the exam"?

You're looking at the wrong problem and numbers.

If you score 70% in an exam, you are not very accurate.

If that was the only exam on which you scored 70%, and in all your other exams (of which there were more than ten) you had scores better than 95%, then you tend to be highly accurate, even though on that exam you were not accurate.

In other words, the claim by kaetl is that on average, some particular belief about group difference will probably be very accurate, because most of them are, but there are some that are not accurate at all. Which is why they tend to be highly accurate, but they're not always highly accurate (or even accurate at all).