That’s a good question, sometimes I believe it’s because other medications could be interfering with its mechanisms; also not to account for failure necessarily, it is to prevent a law suite. If the claim was 100% effective and for some unknown reason someone became pregnant, the company who manufactures and markets that contraceptive could be held liable.
When use perfectly correct birth control pills can prove to be 99% effective but people often mess up their pill schedule. So birth control pills often prove to only be 91% effective because of people taking them at the wrong times or missing days and just overall inconsistency.
