Entire axle has dedicated oil, but, within the axle are two hydraulically actuated wet brake units. While the brake unit is lubricated with the dedicated axle oil, the brake pistons are activated with hydraulic oil via the brake valve.
The brake pistons are sealed with rubber piston rings. Age and heat cause the brake piston seals to wear out. When the brake piston seals leak, hydraulic oil leaches past and mixes, and adds to, the dedicated axle oil.
This doesn't mean that the brake lining is worn out, but, due to the labor involved, it is common sense, and accepted practice, to replace the brake lining while the unit is disassembled.