Had some similar conversation in another forum about the stainless steel oil filters. The discussion was somewhat on how filtering is rated, standards, testing methods.
When a product is advertised at x micron, this may often just be marketing getting involved because that is what people expect to see. The problem is, the micron size is useless information without know the testing method or efficiency (Beta Ratio) of the filter.
For example, I can say my filter will trap 6 micron size particles! The catch is, how many of them will they catch? What you need to know is "at 6 micron, the Beta Ratio is 1.5 (33%)" and "at 25 micron the beta is 10 (90%)".
The marketing people will grab the lowest number (6 in this example) even though many much larger particles will pass through.