I am still not believing that 42-60 micron number has any meaning. Its on one website of one of the marketing arms for these filters and doesn't even say that is the filtering spec. That is literally all it says "micron rating 42~60". Meaningless. All the other filters from the same place have actual specs and ISO references.My question is how if the filter has a 42-60 micron rating? Wouldn't that mean it passes everything below 42 micron?
As for the 99% - most all of these filters use a cellulose material. Its non homogenous. There may be a small number of gaps that are greater than whatever size particle there specifying. So if something hits perfect it may sneak through. Its the same reason why a filter that specs 99% at 30um might actually catch something like 30% of 10um - its all about where in the weave it gets caught.
It may also be to a degree a CYA catch all - in case someone tries to sue, etc.