Answer:
Well, the company must be quite perplexed that there is a 0.1% chance that there is an unclassified type of germ that cannot be eliminated, so they must state that because their is always an individual who finds that Listerine did not, in fact, kill a certain germ (such as E. coli since it is resistant to antibiotics) And they fear being taken to court because 100% is definitely a hard fact to prove and that invincible germs may still exist...
That's all I could come up with off the top of my head. XD
