So there are plenty of ways a filter company can achieve a better ISO result for marketing purposes if they're so inclined. It really comes down to how much you trust the company, and if there's any evidence from independent sources to back up their claims.
This should be a sticky at the top of this forum.
Along with this:
It's not odd to me in my experience for any manufacturer in general to not reach the efficiency claims they make. Changes in model name, material suppliers, material countries of origin, changes in production facilities, changes in brand owners, manufacturing machine adjustments, new employees, new management, poor QC/QA control practices,....I could go on and on with respect to this topic. Most manufacturers know this, and now you do to.
And my trust in these companies is very low. Tears and leaks are either not known about (incompetence) or ignored (fraud) by several major manufacturers (Purolator / Mann & Hummel, WIX, Fram / Champ Labs)