Cause when you do just a standard pan drop and filter change you dont get all the fluid out, some stays in the torque converter. Its normal for fluid to get very minimal amounts of metal shavings in it. Most sensors are essentially magnets in the transmission, so when you get to much shavings, itll stick to the sensors and make them faulty. Its normal. You may have a bad OD sensor too. But the 2 stage flush drops the pan, you add new fluid, then you cycle it thru the old filter instead of clogging up a new one, then drop and run new fluids in it. Make sense, kind of jumped around there a bit!