Slippage on Forexfx

I traded on October 11th 2019 on Forexfx at the gold price of 1499.38. I didn’t set stop loss. Later the market was going down, so I modified my order and added a 1489 stop loss. The highest price back then was lower than 1489. When the price goes up to 1489, the stop loss was triggered and the final result was 1489.02. However, I checked other platforms and found the highest price was just 1488.31. I questioned Forexfx, who only responded me with a “Thanks for you feedback. This kind of situation is normal. It has nothing to do with slippage.”