"All are not. . ." or "Not all are. . " What's the difference?

"All are not. ." means that not even one is. "Not all are. . " means that at least one is, and at least one isn't. I keep hearing ads saying that all are not(whatever) when obviously what is meant is that not all are. Otherwise the advertiser is saying that his product won't do what he is advertising it for under any conditions.
In that case, why is he wasting his money to advertise it, and why would anyone want to buy it?