Underestimating the noise intensity by only 3 dB means doubling the noise to which the user is exposed. An even larger error could devastate the eardrums, causing permanent hearing damage.
That's why it's important to understand the NRR - how it works, how to calculate it, and why you should derate it.
The NRR isn't a straightforward measure and is not perfect by any means. When evaluating noise protection options, it's best to err on the safe side. Don't rely on the number on the packaging. Derate it and add extra protection if you need to.