People in the trucking industry, can you explain what the "Variable Milege" payment is for? My husband pays?

...for this variable mileage at a rate on 12 cent/ mile. That's around $2000 a month out of his pocket. He is a lease operator. Owner operators for the same company don't pay this. Anyone know about this? It seems unfair to pay on average 500 a week. He will soon buy his truck, but in the meantime, what has $24,000 a year been going to? Why do many other companies not charge this fee?