In a 5-star rating system, people seem to think 5 stars = 100%. But since you cannot leave 0 stars, the star-to-score relation is actually like this:
1 star = 0 - 20%
2 stars = 20 - 40%
3 stars = 40 - 60%
4 stars = 60 - 80%
5 stars = 80 - 100%
So, 5 stars doesn’t mean perfect, it just means an A.