when you say "I saw round completion to go much above 300 percent" that is call 'Round Variance'
What's the 'Round Variance'?
'Round Variance' is a calculation made taking into account the block difficulty and the effort already made by the miners to find a block, each time a block is found the round is restarted. A 100% variance means that the expected effort to find a block was already reached, but it's not deterministic, in other words, it's not guaranteed that at 100% a block will be found. The process of finding blocks is very similar to a lottery, where each miner makes several bets in order to try to crack the hash code, thus sometimes when we are lucky blocks are found early with just a few tries (before 100%) or unlucky (after 100%).
soo "The process of finding blocks is very similar to a lottery" aka high % 'Round Variance' bad luck now it depend how pool it is set or make to work & for this only a admin can say it or give details
when you say "I have seen my expected coins to drop completely to zero" that it happen because your portion (%) of share vs all share found it get verry small let say 10,000 share vs 1,041,915,720 aka you = to 0 what it can be happen let say on a Round Variance ~480% & your income it will be 0
well technical is not a scam but is a perfect scheme to rob miner
abuse 'Round Variance' system aka you can mine full 12 hour 120% variance you stop miner various reason (shutdown pc, restart, update so, driver ... move to other pool ... ) & you not start mining agen BUT round it goo further to let say 400% well all your time will be invalidated, nullify to 0 or to very very very few small pay
low % round variance it can happen to
aka let say 20% & amount of payment it can be low or high depend by h power what you have but usual all it get some payment vs high round when many can get nothing
from what i understend high variance it can happen, is fine but when that is in row soo many time over & over then is a problem & only admin of pool can say what is wrong but usual none of them will replay
or they will say bad luck ... we are sorry for your loss & that it or to low h power ... what you or miner can doo is nothing or find pool what it work best for each of us like h power we have
Example for how bad thing can goo:
Block Validity Finder Time Variance %
XXXXXXX Confirmed X 2017-08-18 11:56:04 345.66
XXXXXXX Confirmed X 2017-08-17 21:20:15 168.29
XXXXXXX Confirmed X 2017-08-17 14:14:23 484.61
XXXXXXX Confirmed X 2017-08-16 19:18:30 322.17
XXXXXXX Confirmed X 2017-08-16 06:35:59 97.32
XXXXXXX Confirmed X 2017-08-16 02:58:04 225.26
XXXXXXX Confirmed X 2017-08-14 13:08:09 206.78
XXXXXXX Confirmed X 2017-08-14 00:11:25 179.01
XXXXXXX Confirmed X 2017-08-13 07:58:56 437.59
XXXXXXX Confirmed X 2017-08-12 09:43:22 445.07
i am new to & in case i make some mistake some older miner can correct me & hope it help you to undestend way what you was describe was happen & help to minimize your future loss
cheers