Steve nailed it. As the MAF sensor is slowly gumming up the ECU will be continuously compensating and the car just happily drives along. You disconnect the battery and it goes back to factory default, I.e. expecting a pristine MAF sensor. In time it will adjust to the dirty MAF again, but better is just to clean the sensor and throttle body.
Don't expect the car to run great immediately after cleaning the MAF, though. The ECU still has to deal with an old sensor, it will take some drive cycles for it to adjust properly.
BTW, if you find a lot of oil in your throttle body thern that could mean your AOS is on its way out. Rather cheap to fix.
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