Because you're still paying more than you should!
You are missing out on regional differences in wholesale fuel pricing and competitive pressures between truckstop chains. Consider the example below, showing real market pricing on a single day across Ohio.
Truckstops in the same chain had greatly different pump pricing in different locations, and there was significant difference between chains for stops in the same location. No matter what your negotiated discount, if your driver filled his tank in Cincinnati instead of Cleveland on that day, fuel probably cost you $0.17 per gallon more than it should have. A truck getting 6 mpg, driving an average of 10,000 miles/month, would have cost you more than $280 in excess fuel expense with those lost savings.
How many trucks are you running each month without IDSC ExpertFuel?