You guys are both right, but talking past each other.
Lysanderxii saying is that the NRR of different types of plugs varies so much that there are SOME plugs (with very low NRR figures) that when combined with muffs, will have a lower combined NRR than OTHER plugs (with very high NRR figures) when combined with the same muffs.
e.g.
NRR 22 plugs plus NRR 22 muffs could provide less protection than NRR 33 plugs plus those same NRR 22 muffs.
You are correct that when you add a given set of muffs to a given type of plugs, the resulting combined protection will always be higher than if those muffs or those plugs were used by themselves.