How can the demo account for backtesting be different from the current account?

 
How can the demo account for backtesting be different from the current account? And it's very different, can anyone tell me why?
 
The one is real world and other one is dream world.
 
Gouxiang Lan:
How can the demo account for backtesting be different from the current account? And it's very different, can anyone tell me why?

demo trade

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