There are a couple of reasons this could happen. One is that cash payments are paid out on the gross amount, not the net, so that when an end-user purchases virtual currency using PayPal, credit card or other direct payment options, a small percentage of the total transaction is withheld to cover processing fees. Also, in some cases our customer service team will issue virtual currency in order to keep the end-user happy even when we aren't paid by the advertisers, as could happen if a pixel misfires but the end-user shows valid proof of having completed an offer.