Case in point - prison population. If 10% of the population is a specific ethnicity that nation's prison population must be 10%. If it is more or less the criminal justice system is prejudicial.
It should only be that 10% if everyone commits crime at the same rate. The trick that politicians especially do is have people focus on the wrong metric, because most people don't understand stats.
The prison population should reflect the population of those committing crimes, which is likely different to the general population.
There are very few systems where you would expect the makeup to exactly match that of the general population, unless it was a true random selection. There is always some form of self-selection going on as an input to most systems.