Why do disinfectants always claim 99.9% against germs and viruses but not 100%?

Mutation rates vary, also a lot of surfaces have nooks and crannies that the cleaning doesn't reach. 

Because bacteria growth and reduction is considered in log reductions. A three log reduction is a 99.9% reduction in the bacterial load present. But here’s what that really translates to: if there are a million bacteria present on a surface and there is a 3 log or 99.9% reduction, there will still be 10,000 bacteria present on a surface after cleaning. They can’t claim 100% because the effectiveness depends on how many bacteria are present to begin with. If you have a lower bacterial load present on a surface to begin with, that 99.9% effectiveness could essentially kill all the bacteria present. But if you have a highly contaminated surface there still could be a lot of bacteria present even after using the disinfectant. So they can’t claim 100% effectiveness because that wouldn’t always be true.

Short answer: NOTHING is 100% effective in reality.

This question has nothing to do with personal finance.

It's to avoid lawsuits when the product is proven to be ineffective.