Why do periods still occur while on birth control? my understanding has always been that a period is the unfertilized egg exiting

Withdrawal bleeding: First, some women stop having bleeding on the pill. The way pills are set up (a couple exceptions) is 21 days of hormone and 7 days of either no pill or a placebo. The bleeding during that week if from the withdrawal of the hormones. Women who use continous pills (starting new pack day 22), are more likely to stop bleeding at all. There are also 2 pills on market with withdrawal every 84 days.