If a study determines the difference in average salary for subpopulations of mechanical engineers and civil engineers is NOT significant, then the subpopulations of mechanical and civil engineers are not earning different salaries. Study determines that there is not any significant study so thats why both are not earning.
Or, if you're familiar with the empirical rule and the basic properties of the normal distribution, you can use the fact that approximately of the distribution lies within two standard deviations of the mean, or
which means about of the distribution lies outside this interval, or