Why Is the Theoretical Profit of a Box Spread Equal to the Difference between the Strike Prices?

A box spread is constructed to be a synthetic loan or deposit. It is a combination of a long synthetic future at a lower strike (K1) and a short synthetic future at a higher strike (K2).

The payoff at expiration is K2 – K1, regardless of the underlying asset's price. The theoretical profit is the present value of this difference, as the initial cost of the position should be the discounted value of the certain future payoff.