[smeej]

This is an important point. The failure rate measures, "If 100 people otherwise capable of becoming pregnant have an average amount of otherwise potentially procreative sex for a year, how many of them will get pregnant?"

Nine out of every hundred is a LOT to start out with. And if the "average amount of otherwise potentially procreative sex" part increases by, say, 11%, you get one new pregnancy out of every hundred.