yesterday i saw alot of jiken yesterday. at NSK. kampung jiken also have. 

It's the same for any type of oil. In any machinery, the lub oil or the oil it used to make the machine function will degrade overtime, due to heat, moisture, friction at cold start, friction when the gearbox is running (in real world even you have lub oil there will still be friction between gears is just how much % of the friction is reduced with lub oil, it's not possible to have 0% in real world.). In my factory I have 30+ machines all have gear oil, and it gets dirty and becomes black after 10,000hrs of operation. All my gear box are enclosed and with oil cooler.
Don't forget inside AT gearbox you still have planetary gear set. Although your gearbox is sealed but it is not in negative pressure (vacuum) 

The real problem starts if the oil is not change on schedule, wrong grade of oil or they have lost their lubricating ability. Then you will start to have larger chips of metal debris because the gears are grinding against themselves and the opposing pressure on the gear teeth will cause the teeth to break apart as the uneven heat stress builds up within them.