Eh, it's just the nature of the ear pro itself. The Peltor ComTacs (both the III and VI) have about the same NRR as the Sordins (20 NRR for the ComTacs), just due to how low profile the cups have to be. The Peltor Sport Tactical 500 is a much bulkier muff, so it stands to reason that it has better protection.
This is why I doubled up with the SureFire plugs when I used Sordins and TCI Liberator HPs, and also partially why I switched over to the NFMI-enabled Ops Core AMPs. The ComTac VIs have the method of having an earplug mode, which appears to be an extra loud mode to compensate for people doubling up.
Beyond that, NRR in it of itself tells an incomplete story as to the total protection provided:
https://trevoronthetrigger.wordpress...rr18db-rating/