An order may be not executed at the set price if there is gap. The gap is a price break that occurs when prices sudden change. The gap means that in the market there are no buyers and sellers that are ready to make a deal at a price that exists within the gap. If a trader has a stop order at a price inside the gap, this order will not be executed at this price, because in the market there are no buyers and sellers at this price. In this case, the order is executed at a gap price, i.e. at the price that sellers or buyers are ready to pay.
For more information, visit the section "Trading conditions / Forex Order Types".