In general, when we're talking about birth control methods being ineffective, it's because those methods fail or are used improperly. It may help to know a bit more about how effectiveness rates are calculated! Our article
Have a Little Faith in BC has some in-depth information on this process that you may find helpful, but here's the most important excerpt:
"Effectiveness rates of contraceptive methods are all figured based on one full year of use, not for each single incident of sex or use of a method. Perfect, or proper, use of a method means that in one year, that method was always used, and always used following the directions for that method to the letter, without any errors or mistakes. Perfect use of most methods in one full year is unusual, except for methods like the IUD or implant where a user does not have to do anything. Typical use means that in one year, sometimes a method has not been used according to the directions, or was not always used. Typical use is called that because that is most typically how methods are used in a full year, since people aren't perfect, and most often, in a year, will tend to make a mistake now and then with a method."