It has an upper limit below 100% PPS ... but that's the absolute limit ... what you get can be anywhere from that or lower (never above)
When I realized this, I moved away from CPPSRB... stated more simply, with CPPSRB, you will NEVER make more than 100%-fee over any time frame. With PPLNS, you can make more than 100% over a limited time frame. Put even more simply...
- Under PPLNS if the pool does well (luck wise) over any given time frame, you will also do well (payout wise). Also if the pool does poorly (luck wise) over any given time frame, you will do poorly (payout wise).
- Under CPPSRB, if the pool does well (luck wise) over any given time frame you will simply get paid shelved shares/back pay, but never more than 100%. If the pool does poorly (luck wise) over any given time frame, you will do poorly (payout wise) until the pool has better luck, then you might be "made whole" (in terms of the amt you are owed based on comparison to pure PPS). I believe that theorhetically if a CPPSRB pool has better luck than 100% over its lifetime, then the result of that good luck goes to the pool operator. That's how PPS works anyway.
On a long enough time frame or with enough hashrate (any technical issues notwithstanding), PPLNS (and really all payout calculations) will be expected to trend closer to 100% minus fee. This includes CPPSRB but it will only from approach from below 100%.
For an analogy, think of it like golf. Hitting a block at 100% expected difficulty is like shooting par. With CPPSRB you can shoot lower than par, but you will never be rewarded for it. If you shoot higher than par, you are penalized for it. With PPS you will always be rewarded at par-level no matter what you shoot. With most other payout calculations, you are rewarded for shooting lower than par and penalized for shooting higher than par.
Sorry for getting so OT with this, but hopefully it helps anyone trying to figure out these payout calculations. The real question you must ask yourself is "Do I feel lucky?" ... Well, do ya, punk?