They know this because the people who develop IQ tests know how many people in their norming sample were able to achieve at each level, and they have calculated the scoring to follow a normal curve with a mean of 100 and a standard deviation of 15.
I can testify to that. I took the new GRE yesterday, and they could not give me my score instantly like they normally do, because we are the population (the "norm", normal distribution, the bell curve) they will base the scores on. After they have enough of us real people types who have taken the test to know how our scores are distributed, ETS will tell us our scores and base the next batch of scores on how our scores were distributed.
My friends are very amused to think that I am now the norm.
