Decrease in Cholesterol

A doctor is studying cholesterol readings in his patients. After reviewing the cholesterol readings, he calls the patients with the highest cholesterol readings (the top 5% of readings in his office) and asks them to come back to discuss cholesterol-lowering methods.

When he tests these patients a second time, the average cholesterol readings tended to have gone down somewhat.

Explain what statistical phenomenon might have been partly responsible for this lowering of the readings.