If the $108 total restaurant bill is divided evenly, $27 is the average cost per diner. This cost-averaging approach treats each diner the same. When costs are averaged across all four diners, both Emma and Matthew are overcosted, James is undercosted, and Jessica is (by coincidence) accurately costed. Emma would probably object to paying $27 because her actual cost is only $15.
Broad averaging often leads to undercosting or overcosting of Products or services: