Once a stop-loss order is executed, your position will be closed at the first available price. Therefore, the position may not be closed at the specific loss you have set up due to market conditions such as slippage.
To avoid this, you can use a guaranteed stop-loss. In this case, your position will be closed at the specific price you have set regardless of any market conditions such as slippage. Please note that a guaranteed stop is available for an extra fee.