Most of this discussion assumes a mined coin is exchanged for the payout coin. The 20% reduction also occurs when the matured block
is the same currency as payout. In such cases there is no exchange. There's no use adding unnecessary variables to the problem.
I wish you success in finding the problem.
The theory is that the coin is exchanged behind the scenes from X to BTC and back to X for payout. Could very well be but the loss is too great and too consistent for that IMO.
Agreed, their could be a double exchange but my point is the problem occurs when no exchange is needed. The single coin
case is a better one to analyze as it may bypass all the exchange code. Even if it doesn't bypass the exchange code the single
coin case is easier to observe because there is no exchange rate fluctuation to be concerned with.
It is also my understanding that zpool does not exchange for altcoins on the open market. All altcoin payouts are done from
mined coins. So any non-BTC payout does not require a market exchange.