There's a very strong correlation - if you don't think that's the result of (racist - conscious and or unconscious) attitudes in the workplace, please suggest an alternative hypothesis.
Correlation doesn't always mean causality. Post hoc ergo propter hoc. That's policy 101.
There are a number of reasons that could be in play apart from racism.
Qualifications are strongly predictive of earnings, and certain groups achieve qualifications at much lower rates than others.
Earnings are inversely associated with fertility, as time out of the workforce damages careers and earnings potential, as does working part time, and certain groups have higher fertility rates than others.
Average earnings are higher in some areas than others, eg major metropolitan centers tend to be higher than rural areas, and certain groups are disproportional rural.
Earnings tend to rise with age, and some populations are (on average) younger than others.
There are many potential factors at play. Jumping straight to hand-waving "racism" claims without proper analysis is sloppy, and makes for poor solutions.