Lots of top schools use a 5.0 system to determine grad rank and valedictorian. Your cum never goes above 4.0 but internally your classes are scored on difficulty above 4.0 . An advance law class might have a potential 4.3 for example. Columbia law grads are highly sought after by major law firms and they wan the highest ranked students. If 100 people have 4.0 they need to be able to differentiate based on classes taken.