First off, we should note that both of those efficacy numbers assume what’s called “typical use.” That means they take into account a typical amount of human error. For example, if you took your pill at exactly the same time every single night, hormonal birth control pills would be 99.7 percent effective. That’s perfect use, and it means for every 100 people taking the pill for a year, there would be an average of 0.3 pregnancies. But humans are fallible. They go to parties and travel between time zones and just plain forget, so “typical use” offers a more realistic view of contraceptive efficacy. Pills can be 99.7 percent effective, but are just 91 percent effective the way most people take them.