Why are so many things 99.9% effective? Why not 98.9%?
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The methods we have for [sterilizing](https://www.cdc.gov/infectioncontrol/guidelines/disinfection/efficacy.html) things (chemicals, radiation, dry heat, steam, filtration) are never 100% effective, they only cause a *reduction* in the numbers of bacteria. Typically for “sterile” results, methods produce a reduction of 10^12 or 10^9 and there’s “bioburden” testing to prove that the material starts with much fewer bacteria than that. For testing, they usually put in a spore strip with 10^6 bacteria, and run the sterilization method at half duration or half strength, to “prove” that it still kills everything in that spore strip. For “consumer grade”, your 99.9% is actually the equivalent of a 10^3 reduction, and they just write it as a percentage so they don’t have to explain to the public all this stuff I just wrote.
In addition to these levels of “effectiveness”, the testing itself is a statistical testing. You have to open the package to “test” an item, and that “destroys” the item in that it’s no longer sterile and you can’t sell it anymore. So they can’t test *every single item* in a lot, they can only test a few samples. So the answer is a [statistical answer](https://en.wikipedia.org/wiki/Sterility_assurance_level), we’re 99% sure this whole lot is sterile, but 1% chance maybe one of the items is not sterile. So legally they can’t claim 100% if they’re only 99% sure.
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