Why is the price of the order execution different from the Close price on the chart?

The Orders are Executed at the Ask and Bid Prices, which are displayed in the top left corner of the chart while the chart data is built at the middle price of the market’s Buy and Sell. 

To enable the market price buttons:

Open Settings (gear)-->Trading-->tick Show Buy/Sell Buttons

Or 

Open Settings(gear)--> Scales-->tick Bid and Ask Labels to see them on the Scale. 

Buy Order executes at the Ask Price while  Order executes at the Bid Price.

 The Long Position, on the contrary, closes at the Bid Price and the Short one at the Ask Price.