Why are some 'HEPA' cabin filters so much cheaper than others?

True HEPA requires meeting a defined particulate standard (Bosch cites ASTM D2986 / ISO 29463-3 in its product copy). Filters that market 'HEPA-like' performance without citing a standard are usually high-efficiency paper, not HEPA. Specialist forum threads we read (driveaccord.net, subaruoutback.org) note that genuine 99.97%-at-0.3-micron performance is what causes the airflow restriction — if a 'HEPA' filter has no airflow penalty, it's probably not actually HEPA.