If birth control is used perfectly, it can be 99% effective. Perfect use means that the birth control is used every day at the same time, without any exception. Typical use is how it is most commonly used. It means that the pill is taken at different times or a day is accidentally missed. Birth control, when it is typically used, is 91% effective.
I have read this information from here
http://www.pregnancyweeks.org/can-you-get-pregnant-on-birth-control-pills/ where it says there are 4 most common things that reduce birth control effectiveness - Skipping Pills, Taking pill in different time, Taking certain medications and using herbal dietary supplements like St. John's wort