The results they post are simply too good to be true. They claim to have 10 people working for them out of a Geneva office. Just add up the costs for such an operation, in one of the most expensive cities in the world. Then consider that a 2% difference in exchange rates between different exchanges doesn't mean a 2% profit. Since you need both fiat and BTC to make the arbitrage-trade, your profit is already down to half that of the price-difference. Then you need to keep reserves of both fiat and BTC on all exchanges, while not all reserves are used every day.
Often price-differences are predominantly one-way: For example, BTC-e is almost always cheaper than Bitstamp. So it is necessary to wire USD from Bitstamp to your bank account and onward to BTC-e, which can easily take over a week. So a large part of the funds is sitting inactive or is locked up in wire-transfers. The price-difference required to achieve the stated results is several times higher than the actual profit figure. And that's not counting all their operational expenses.
The price difference yesterday was always at least $15-20 and they posted .5% returns. The reason for this is, they didn't have fiat in BTC-e at the time, to take advantage...presumably because of fully taking advantage of an opportunity on Tuesday. So they had to wait on wire transfer of funds back to BTC-e and missed the chance yesterday. Many days, they report having earned most of their gains from the smaller exchanges, like Kraken and Vault of Satoshi. So far this month, they have only cracked 2% twice. That's not "unbelievable" gains from arbitrage trading. They're great gains in comparison to any savings account you can find, though.
Also, they do take advantage of days where the major exchanges have price parity, to rebalance their accounts by transferring Bitcoin between them and avoid the delays and costs of wire transfers.