What is meant by "standards of justice have not always been met"

Sometimes people often are accused of something they didn't do. Invalid evidence and a crummy jury could put an innocent person in jail or possible death row, while the real suspect is on the loose!  

Our nation was built on the concept of equal justice for everyone. However, blacks were enslaved and later faced discrimination, lynching, and unfair treatments. Other minorities have also been unfairly punished. The Civil Rights Act was one example of trying to reach goals of justice.