As expected unless you hit some 1M+ shares...
LOL, 17% less than PPS? .. I dont think thats, "expected" .. "expected" would be hovering around 100% pps.. and deviate from that up n down .. if I can maintain a 117% pps for just as long a period, then I will retract my statement that the formula is broken...till that time, its broke =P
The Pay-on-Target payout distribution is highly asymmetric. You don't get roughly equal numbers of above-average and below-average shares; rather, you get (with a = 0.8 ) 87% of the shares to be lower than average, then once in a while you hit the jackpot. Maybe you will soon get that big win that will bring you back to the black, maybe not.
This of course does not rule out the possibility of an issue with the implementation.