While the High and Low Synthetic strategies are both hedged positions, the futures leg is subject to a daily cash variation of the mark-to-market gain or loss whereas the stock leg is not (mark-to-market gain or loss is reflected in account equity but there is no cash impact until the position is closed).  If, for example, an account holds a High Synthetic position and the stock prices increases significantly, the resultant variation pay on the short futures leg may erode the account’s cash balance resulting in a debit balance which is subject to interest payments.  The net effect in this example would be to reduce and potentially erase the earnings on the High Synthetic position