The size rating is the smallest size particle that the filter will let through. The filters are made by a felting process where the fibers are laid down at random and pressed together.
This means that there are millions of smaller nooks and crannies that trap smaller particles. Filtering out these tiny particle can save your tools and equipment for a lot of damage. Many people think that only large particles can damage machining equipment and tools, but small particles often do the most damage. See our article on the benefits of filtering machine coolant, or take a look at our filtering section for more articles on this topic.
Instead the filters are laid out overlapping at random
Dirty coolant has lots (10,000 - 25,000 per cc or 165,000 - 400,000 per cubic inch) of different size particles.
The bag is 0.125" thick and the cartridge filter is 1.5 inches thick. 25 microns is 0.000985 inches. 126 layers in the bag and 1,522 layers in the cartridge.