The difference is in the transportation from the dairy to packaging plants.
Tanker trucks carry it. They might carry a load of Benzene, Gasoline, or whatever, then on the back-haul pick up a load of milk.
It costs them less to transport it if they don't wash out the tank between loads. Therefore, the cheap milk comes from tankers that don't get cleaned out, the expensive milk comes from tankers that are cleaned & sanitized between loads.

Just kidding.