Can someone explain how Consumer Reports ranks cars?

Specifically, this part: When viewing consumer reports online, they rank cars by the "overall test score." However, they also have three tiers, the highest being a circle with a checkmark in it, the second highest being just the checkmark, and the lowest having nothing at all. The "overall test score" doesn't necessarily coincide with the checkmarked tiers...meaning that there are third tiers ranked above 1st tiers in the overall test score. Could someone explain what the difference is?