Can you tell me for the rhythm method of birth control, why do you minus 18 days from your shortest cycle and 11 days from you longest cycle?

Difficult to say: If you have a very regular length between the first day of one bleed to the first day of the next, then you ovulate 14 days prior to the onset of bleeding. If that interval is actually not regular, then no system is going to accurately predict ovulation. Any system that is just estimating it in the setting of irregular cycles is going to be at least somewhat inaccurate.