The stats are correct but the return is almost always lower on zpool than on the other pools that doesn't autoexchange.
ex:
skein
zpool.ca 24hour payout per ghash: 0.00449BTC
yiimp.ccminer.org payout per ghash: 0.0062BTC
+38% if you mine on yiimp and exchange your coins yourself.
But zpool is more profitable than the nicehashminer..
so you "say" its stealing 20% profit.. and still it outperforming nicehashminer must be doing something right...
you cant compare a marketplace with a pool
what you can compare:
mine coin X on zpool and another pool with same hashrates, dont exchange to anything.
zpool payout coin amount for a given timeframe will be ~20% less than from other pool