Comment by fabian2k

I've never thought about air filters, but the explanation on why they also filter smaller particles is very similar to size exclusion chromatography, a very common method used in a biolab. This is also a method that might appear counter-intuitive at first.

The idea there is to separate molecules according to their size. So you press them through a column of porous beads. Small molecules can enter these pores, which delays them and they travel through the column slower than large molecules that cannot enter them. This is pretty counter-intuitive, especially as other similar methods work as you'd expect with smaller molecules being faster to move through the material because they don't bump into it as much as larger molecules.