It’s important to understand the difference between how stop orders and limit orders are executed, why this difference exists.
When a stop order triggers, it becomes a market order which is filled at the best available price in the market. Stop orders are triggered in this way as a safety measure to protect your trade from further losses. If stop orders did not fill immediately then you would run the risk of further losses if your trade remained open and the market continued to move against you.
Limit orders on the other hand can only be filled at the specified price or better. That means they will be filled only if there is sufficient liquidity at the specified price or better. Limit orders offer price certainty, but not execution certainty. Market orders (including stop orders) offer execution certainty, but not price certainty.
As you can see from the data below, when you look at all order types across all FXCM accounts, positive slippage occurs just as frequently as negative slippage.
Note that positive slippage is most common with limit orders, while negative slippage is most common with stop orders. That has to do with the direction in which prices are moving when these order types are triggered. Limit orders are triggered as prices are moving in favor of your trade, while stop orders are triggered when prices are moving against your trade.
When trading forex as with any financial market, your capital is at risk. This page has more information on how you can maximize positive slippage and minimize negative slippage.