essure

i've been seeing these commercials for essure, a female sterilization method for permanent birth control. so i checked it out. from their website:

The Essure procedure is 99.80% effective based on 4 years of follow-up. Essure is the first and only FDA approved method of female sterilization. There were zero pregnancies in the clinical trials.

if there were no pregnancies, then how is it only 99.8% effective? did someone die?