The 99.9% effective claim discussion has been aired on this site before. It is used because no absolute (100%) can be guaranteed, particularly so in households where air borne pathogens are always coming into contact with surfaces but this applies to almost everywhere humans or animals are.
What the claim purportedly raises is that the active ingredients are very effective and the claims use what may be termed puffery which again has been raised on this site (Truth in Advertising, should we accept Puffery). Basically it means the Law as it currently stands allows them to get away with it.
