Standardized exams: a simple question

"If you are able to accurately state that standardized exams have differing results by race due to bias in the exams, why can't you isolate the bias, and create an exam without this bias?"

Many years ago, I was able to see noted sociologist William Julius Wilson when he spoke at my university. The topic of his speech ventured into the subject of standardized exams, and I asked this question. I don't remember the exact details, but I remember that even he didn't have an answer to this question.