Why was my Stop order filled at a different price?
The following types of orders may be executed not at the declared price: Buy Stop, Sell Stop and Stop Loss.
Stop orders are pending orders that are triggered when the Ask or Bid price of an asset reaches the price specified in your Stop order. Once the Stop Loss, Buy Stop or Sell Stop order is triggered, it becomes a Market order and gets executed at the current Market price available at the moment.
This is the reason why there might be differences between the price specified in the order and the execution price.