The real scenario of Google AdSense

Do you know how much people generally do income from Google AdSense? I have some friends who are using Google AdSense program more than 1 years ago. I was first introduced to Google AdSense program by my friends. They have several web sites and blogs. They earn money but not huge. In the following table, I will present my four friends income. The following earnings are not my earnings. Those earnings are my four friends earnings. They use only Google AdSense program and earn like: