There are two issues that cause the backtesting not to reflect reality. 

1) There is slippage, the backtesting tool assumes trades are made at 4:00pm on Friday using closing prices. This is impossible as the information is not available...

It depends on the how often the trades are made and the bid/ask spread on the stocks you buy.

Lets assume you do weekly trading, but do your buys and sells at around 11AM every Monday.

Backtesting uses the close price as of Friday...


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