Try emailing the site owners. They have been pretty straightforward so far and might be able to clear it.
To me it looks like 3.5 BTC is missing. If you add up all the transactions that were paid out you get a total of ~2.7 BTC, but the wallet received 6.2 BTC.
If you look through the transaction list you will see that some addresses are getting paid multiple times for only a single transaction.
The address: 19fpsJn8hdpuPkqoK1pDQgpr2HurpXH8r1 for instance got paid 12 times for only 4 deposits. That's 3 times the amount they should have been paid. Ref:
https://blockchain.info/address/19fpsJn8hdpuPkqoK1pDQgpr2HurpXH8r1There are a lot more inconsistencies with this site. OP you need a new coder, your current one has no clue what the @#$! he is doing and is losing everyone's money.