I read on https://eardefender.co.uk/snr-vs-nrr/ (mirror):
SNR (Single Number Rating) is the EU’s standard for showing the attenuation of hearing protection. [...] NRR (Noise Reduction Rating) is the noise reduction standard used in the United States. [...]
The most obvious difference between SNR and NRR ratings is that SNR ratings tend to be a few decibels higher. An exact formula for converting NRR to SNR doesn't exist, but SNR ratings tend to be about 3 dB higher, so a product with an NRR of 30 will have an SNR of about 33. This is because the two ratings are determined by following different testing procedures.
Why do SNR ratings tend to be about 3 dB higher than NRR ratings? I.e., what difference(s) in the testing procedures account for this gap?
I note that sometimes the gap is much higher than 3 dB, e.g. this Mpow earmuff (mirror) is rated SNR 36dB and NRR 29dB (= 7 dB gap between the NRR and the SNR).