It's different because you have no idea whether you'd really get execution when and where you want it. A few seconds could make a difference. The size of your trade--puny compared to program trades--could make a difference.
You might not get hit at all. In live trading, you'd then have to decide whether to wait, or to adjust your order or let it go entirely, and so on. A great many more factors are involved in real life than when you're just watching a chart saying: I'll buy here and sell there.
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