After a recent presentation of health plans at her company, Julia Brouck joined ABC PPO. The next week she made a visit to a pediatrician in the plan with her son, Arthur, and daughter, Emmy Lou. After seeing the pediatrician, she was told that there was a $15 co-payment for the visits. "I don't understand," Julia said, "I thought all physician visits were free." Explain the potential source of this misconception.