I started this thread which has caused all sorts of confusion and I will now offer a possible explanation. Below is another photo that Kevin sent me that I didn't post at the time. This shows the BIJOY more side on:
Bearing in mind that the light is still poor, I think that the top of the mast is darker than the bottom. If I then zoom in I think this is because the bottom is shrouded in engine smoke from that front facing vent.
I think the original photo has the whole of the top of the mast in smoke and poor light.