Why does this income thing bugs me so much?

It would be wise to not tell anyone about how much money you are making. The best thing you can do is show your achievements in ways that do not involve their knowing your personal business. Do not worry about being an underachiever, you should know that if you are really making 6-8K as you say in your question. Do not worry about other people and what they think of you, just do the best you can.