Why would a different customer have paid a higher rate than WEPCO to Union Pacific for the transport of resources or other products?

In 1999, Wisconsin Electric Power Company (WEPCO) contracted with Union Pacific to transport coal to WEPCO from mines in Colorado. The contract required WEPCO to notify Union Pacific monthly of how many tons of coal (within a certain maximum) it wanted shipped the next month. Union Pacific was to make “good faith reasonable efforts” to meet the schedule. The contract also required WEPCO to supply the railcars. When WEPCO did not supply the railcars, however, Union Pacific used its own railcars to deliver 84 percent of the requested coal. Claiming that the minimum percentage should have exceeded 90 percent and that Union Pacific was shipping less because other customers paid higher rates, WEPCO filed a suit in a federal district court against the railroad for breach of contract. The court issued a summary judgment in the defendant’s favor. WEPCO appealed.