CPPSRB has other "fairness" attributes too. With backpay, you have the potential to earn full PPS for every share you've ever submitted. PPLNS throws away shares after a given time frame; you will never earn anything for them. I suppose that if the "N" in PPLNS is large enough, you get kind of the same thing (since shelved shares that are old enough are unlikely to ever be paid out).
No, even if "N" is relatively small, you can
t still expect to be paid the same as PPS for every share. A larger "N" just reduces variance. I'm not sure what "N" would equate with CPPSRB in terms of variance.
Over time both CPPSRB and PPLNS will converge to the expected value per share. I'm not sure what the difference in variance between the two is.