It's just an ASSumption, pretty much. A lot of stationary power turbines have air intakes designed for heavy particles to fall out of suspension.
These forklifts (powered by propane fueled Vortec 4.3's, BTW) operate in a dirty, dusty yard all day long, and there was a lot of dirt in the filter can that was just sitting at the bottom. The larger of the intake openings leads directly to the base of the plastic portion, so it does not directly expose the filter to the intake stream, but rather the plastic shield deflects it into the vortex section, and then it reaches the filter media.
These lifts are reverse cooled, sucking air through the frame and engine bay, and then forcing it out through a rear mounted radiator, so the filter can lives in a VERY dirty environment, essentially at the front of a vacuum cleaner. The engine bay was RIDICULOUSLY dirty.
The smaller intake opening, toward the other end of the filter, forces the air to run into a plastic dome before it works its way to the filter media, and the cap on that end also held a lot of dirt that could not have reached the filter first, because that section is completely isolated from the filter media.
I was just blown away to see this setup actually used in OEM engineering. Industrial vehicles rarely waste any engineering on [censored]. It's got me thinking. I still need to get some hydraulic hoses replaced, and potentially install a new oil pan, so when I go to the lift truck depot, I'll be sure and ask if they know anything about this design.
Also worth noting is that the engine has a screw-on NAPA 1040 oil filter for a breather filter as equipped from the factory.
