90% efficient at what? Filtering out gravel sized chunks? Unfortunately, there's not a lot to go on here.
However, this is likely a case of where your cousin got what he paid for... or less than he paid for.
I can buy a $3 oil filter, and see the spec is 99% at 30 microns... right on the box. For what I'm trying to accomplish, that's plenty good enough.
Air filters may not be as straight-forward.