As all know, the computation of subtotal will be multiplication of price and quantity
But if the order line has some value in discount column, this computation will differ
Example:
Qty: 2
Unit Price: 200
Discount%: 10
Subtotal = ((Unit Price - discount amount) * Qty)

             = ((200) - (10% of 200)) * 2

               = (200 - 20) * 2

               = 180*2

               = 360

Note: To display "Discount%" in order line, enable "Discounts" checkbox in sales > configurations > settings.