They do not add up to get a new rating. The post mentioning a slight increase is correct.
The advantages of using plugs and muffs with high pressure rounds is to allow the muffs to protect the outer area surrounding your ear (mastoid) from having those violent sound/pressure waves from bring transmitted to your mastoid bone and then to your middle and inner ear where exposure over time can lead to hearing loss. Much of that hearing loss is permanent.
“I know that you believe you understand what you think I said, but I'm not sure you realize that what you heard is not what I meant.” - Robert McCloskey