A 0.3 micron filter will still remove some of the 0.1 micron material. Filtration works in lots of ways. Also, it is likely that the 0.1 micron particles here

A 0.3 micron filter will still remove some of the 0.1 micron material. Filtration works in lots of ways. Also, it is likely that the 0.1 micron particles here have a charge. The 0.3 micron test particle used for HEPA evaluation specifically does not have a charge. Using charged filter media is a common method to capture particles that are too small for the holes / pores in the filter element.