Depending on the direction of the airflow, that could be a good SWAG. Note the "shadow" of clear space below the upper large orange capacitor.
Possibly the clear area at the left is either low airflow of undisrupted airflow that is not dropping dust particles.
Or the voltages on the different parts of the board and electrostatic attraction may have something to do with it.
A smoke test (blowing smoke) with a clear plastic replacing the cover might shed some light on the airflow.
It would be interesting to find out the composition of the dust, starting with whether it is water soluble.