> We always show salary as a normal distribution but maybe it is bimodal.
Who are we? Which salaries?
Starting salaries for lawyers are one of the most famous examples of a bimodal distribution you can find. Salaries across the country generally are less bimodal than that. But salaries shouldn't be modeled as a normal distribution in any event. They skew high very easily, and they can never skew low.
Isn't it more that they match the reality, but the reality only for a relatively small section of a bimodal (or more) distribution?