A Google rating alone can mislead: a 5.0★ from 3 reviews might just be the owner's friends. The MapRank score adjusts each rating for how many reviews it's based on.
Few reviews: the rating is pulled toward the average rating of these results.
Many reviews: the rating is trusted and barely changes. At 100 reviews it counts for half, at 1,000 reviews for about 90%.
That's why a 4.8★ with 2,000 reviews can rank above a 5.0★ with 3.