First, in terms of salary, people with below 80 IQs aren't included because they generally don't have jobs. Second, why would we even have to cut off the tail

First, in terms of salary, people with below 80 IQs aren't included because they generally don't have jobs. Second, why would we even have to cut off the tail portion? Next, you didn't cover that contingency. You keep saying it's all income levels, but it's not. IQ and positive outcomes are positively correlated. Pick a thing, pick a measure of success, you'll find that people with higher IQs will tend towards the successful group. And this is also a "all doctors are tall, not all tall people are doctors" kind of statistic. Higher IQ people will wind up in the more successful groups more often, but being in the more successful group doesn't mean you have a high IQ.