What crackfoo was saying above was that the pool mines more than one coin on each algo (unless that algo only has one coin associated).

That being said:
Your miners are fed workunits for whatever coin the pool  feels is best for your hashrate to work on.  So;  if you have been credited for mining another coin;  the pool will therefore have to mine more of the coin you wish to be payed out in just to fill the hash/rewards that was used to mine other coins on the same algo....

When the other coins you have mined are sold on the exchange, often its fulfilling a open buy order and not creating a sell order;  so the value of the minted coins is usually lower than the current market rate which may be a sell order fulfilled before the buy order is made;  thus the value of your percentage mined was reduced on those coins;  and thus a drop in estimated owed balance.


Hopefully these details help you understand more of whats going on; and why on the main zpool webpage it highly recommends payouts to be in BTC and not an altcoin.