What does ‘statistical bias’ mean?
If you have a bag with 9 white marbles and 1 black marble, when you draw a marble from the bag you would expect it to be a black marble 10% of the time. This is because for every set of 10 draws where you get 0/10 black marbles you would expect there to be another set of 10 draws where you get 2/10 black marbles (if you put the marble back into the bag after the draw). So if you only draw white marbles 1000x in a row you begin to suspect that something is wrong with the bag (maybe the black marble is stuck at the bottom) - or possibly that something is wrong with the marbles themselves (maybe the black marble is hiding!).
In the exact same way, if the US is 60% white, and congress is 78% white - it doesn’t inherently mean something is wrong with any of the people directly (ie. the marbles), but that something is wrong with the system (ie. the bag) - which is causing the discrepancy. Similarly, if the US is 12% black, and the prison population is 34% black, you would suspect that something is wrong with the system first (ie. the bag). Another example: if the US population is 51% female, but only 5% of US Fortune 500 company CEOs are female, there is likely bias.
Now, this could be due to population distribution, economic opportunities, government policies, etc. We don’t actually need to know exactly why to be confident that the results are (statistically) biased. If they were truly unbiased you would expect a correction to occur (ie congress to become 42% white to offset the imbalance or US Fortune 500 CEOs becoming 95% female). However if there is no correction, and the bias persists over time, the odds of the outcome being random are incredibly low and we can be confident of systemic bias.
The only alternative explanation would have to be that, if the system was believed to be unbiased, that something was wrong with the marbles - meaning you would have to believe in eugenics and superior/inferior races or genders (for example, when Nazi Germany thought that Aryan’s were inherently superior and it was the jews who were exploiting and repressing them). This is why if you believe our political systems are completely unbiased and we have no systemic racism - ya might get called racist, sexist, and/or a Nazi every now and then - due to the implications.