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 cant 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.
CPPSRB and PPLNS are very similar. CPPSRB is usually presented as a variant of PPS, but it could as well be presented as a "PPLUNS: Pay per last unpaid N shares".
The difference is mostly for occasionnal miners I believe. In PPLNS, your reward are ONLY affected by the luck as you are mining/shortly after. If you submitted a share just before a luck round: cool! you get great rewart. If you submit it before an unlucky round, boo, you get nothing. After a while, your share is "lost", and any change in luck will nut change it.
In CPPSRB, even if you submit a share just before an unlucky round, and it gets shared, if it is lucky later on, even when you are not mining, it will be payed.
Of course both methods converge to fair PPS value. However, in PPLNS, only luck shortly after you have mined affect your payout, whereas in CPPSRB, any future luck even if you were not mining during the lucky round, will help your payout to converge.