Where does the salary deferral come from? The employee's wages. 

Employee A receives 45,000 wages and choses to defer 5,000. Total wages are 45,000, but W2 box 1 is 40,000. 

Employee B receives 45,000 and chooses to defer zero.

Total company's wages are 90,000, not 85,000.

There is no pension expense here.