The filter medium consists of very fine fiberglass fibres which are processed to a progressively built up filter mat. The fibres have an additional adhesive oil film.  To the pure air side the mesh density increases. This construction guarantees that the varnish particle deeply penetrate into the mesh and increases therefore the absorption capacity.
The rigidity prevents the compression of the filter,  even it is is fully fed up with dust.
So the progressive structure of the filter remains and the full material thickness can be used to increase dust holding capacity.