Remember: after the weekend, the price sometimes opens with a jump from Friday’s. Example. Your stop is at $68, on Friday the price is $70, and on Monday the market opens straight at $65. The price of $68 never existed, so the trade will close at $65. Traders call such a jump a gap.
There’s also a small difference: the price moves very fast, and the trade closes a couple of cents worse than the stop. That’s slippage.