EVERY one of them???
HMmmm...
"Dare a WHOLE lot of assumin' goin' on hyere!"
As I said, there's a bit of oversimplification there (as it would take 20 pages to begin to do it justice), but yes, for all intents and purposes, you can assume that when you've used the 99%-effective spray on 1,000 bugs, 10 will be left alive and cannot be killed by that particular spray.