NRR is the ANSI (USA) rating and SNR is the CE (EU) rating.
The SNR is based on subtracting one SD value from the mean while the NRR is based on subtracting 2 * SD.  That's a difference of about 4 dB.  Then, in the NRR calculation, there is an arbitrary 'minus 3 dB' at the end of the formula for 'spectral uncertainty'.
There's the total of 7 dB.  It can be plus or minus, but it's about 7 dB usually.  So, the equivalent SNR of SnugsGo Shield with a 26 dB NNR is equivalent to 33 dB SNR.