Dear chvv,
You were trading when the news about ECB's decisions to cut the Minimum Bid Rate were released. Immediately, after the announcement volatility of the market dramatically increased and when the price reached your Sell Stop order, it could not have been executed at the requested price because it was not available. Therefore, your order was filled at the next available price
Slippage hardly ever occurs if you trade at XМ. In volatile market conditions (i.e. during market-moving news events, or particularly in active markets) orders get executed at market price if prices have not moved in the accepted latency times. However, when the price has moved significantly, you will be filled at the best price available. This can work out to be either in your favor or against it.
In order to avoid slippages, I advise you to avoid trading during the news. At times, especially when important economic news is released, you may be filled at a different rate than the one you had requested, which is due to a sharp rise or fall in market prices.
Best regards,
Andrey