As I recall, sound pressure doubles with every three decibel units. There is twice as much pressure at 103 db over 100 db.
I wear two sets of ear protection. The first are the plugs rated aroud 20 and then Leighting muffs with a rating of 31. Sure enough, both together work much better than either one alone. What I find the true benefit of wearing two sets is that if I forget to put on my muffs, the plugs keep damage from occuring or certainly not nearly as much damage as going without. Wearing a second set of ear protection is something like carrying a backup.
And yes, the NRR rating is for continuous sound, such as loud equipment like newspaper presses, lawn mowers, etc. and is not for pulses noice such as comes from gun shots. Even so, they seem to work just fine.
Reasons why muffs can offer only so much protection is because the materials out of which they are made, how good of a seal they have to prevent leakage of undampened sound from going straight into the ear, and because much of the sound transferred to the ear drum comes from conduction of the noise through the bones of the cranium. So no matter how good the muffs are and how many layers you wear over your ears, the ear drum will still feel the sound vibrations and that signal will be interpreted by the brain as sound.