Here is a quote from Dr. Hook that explains why he uses 99.99% instead of 100%:
"As a generalization for both you and other readers, you must realize that we VERY frequently get questions asking if different types of test are 100% accurate or if exposures or prevention measures are 100% effective (while the questions are not posed in terms of sensitivity or specificity, but that is what is being discussed). The answer to that is that it is scientifically impossible to be 100% sure. For a variety of complex mathematical reasons far too complex to go into here, all one can do with well conducted scientific studies is estimate probabilities (this is in part because, unless you test every possible person, you cannot be "sure" that you would not have gotten a different answer). By definition, any estimate, cannot be 100% certain. On the other hand, when Dr. Handsfield or I say that something is very close to no risk or of minimal risk, or use any other term to indicate a very small risk, that means "close to zero" in a world where zero cannot be attained. This just as there is chance that you will be struck by a meteorite while reading this, I am very confident that that will not be the case. Can I be 100% confident about this, no but I am sufficiently confident that I can tell you that this is not going to happen."
http://www.medhelp.org/posts/HIV-Prevention/Im-confused--technical-questions-for-you/show/1196601