I've seen this figure before, too. IIRC it's much lower, around single digits, with "perfect use". I think the 20% is with "typical use", meaning all the times people don't bother to use one or they use it incorrectly. For all I know, people in African countries use a condom once a year, thinking it's some sort of magical charm instead of understanding how it's really supposed to work. I guess that would make for a pretty high typical failure rate, too.