it seems that they would like you to explain randomness. This can be easily explained by any normal distribution. Though the mean is in the middle the variation shows that not all people lie on the mean. So taking a random sample may have you lie on either side of the mean. The easiest way to show this is from a sample size of n=1. In this case, unless the person is at the mean, the average of the sample will be on one side or the other. However, as n grows large the mean will begin to move to the middle, and its true value.

Hopefully this answers your question.

-Ryan