Perhaps you are confusing increased income with being higher up on the income ladder? It is true that, statistically, those with higher education do find themselves higher up on the income ladder. They are not making more than they did before, when they did not have higher education, however. Incomes are stagnant.

All we're really observing there is the fact that people range from more to less able, from geniuses who seemingly can do anything to those who have crippling disabilities. At one end of the spectrum you have the people who do well in school and also the workplace due to their natural ability, and at the other, those who struggle in everything they do, be it school or the workplace because of their disabilities. And then everyone else somewhere in between. Statistically, the more able will find themselves higher up on the income ladder, and able to go further in school, thanks to being more able.