Why was my position closed?

If a market trades over your limit level then you will be closed at the best available price at the time. This price will always be at your limit price or better, and is referred to as ‘positive slippage.’

Your stop-loss triggering acts as a request for us to go to market, and close the trade at the best available price. This stop-loss market order can be filled at a price worse than the level you selected—in other words, you'll experience negative slippage.