I know this is really maths rather than English but it has me baffled. “On Rotten Tomatoes, the fourth season has a rating of 100%, based on 25 reviews, with an average rating of 8.3/10. “
Surely an average rating of 8.3/10 MUST be 83%.

Your maths (I love our language differences!) is correct. Not sure what they mean. I've always found rotten tomatoes to be odd anyway. Shouldn't a high rating mean it's bad?