You didn't lose them, the site is pretty clear about how it works:

1. you mine on a given algo
2. you set wallet and payout to the coin output you want
3. the mining happens, coins r mined, then they mature, then site exchanges coins based on rates from the current, live rates from the exchanges listed on the front page
4. you get paid based on the post-exchange

Exchange rates change all the time. Some go up, some go down. Sometimes your balance is gonna go up, sometimes down. If you don't like the # of coins you're returned, try a less-volatile coin.