I meant difference in brightness
I would suggest to find in your place where you live and work a similar geometric situation and take a photo. I guess the result would be the same.
The ceiling has a completely different receiving angle than the walls which are vertical. Is a normal behavior, the vertical areas should be lighter than the ceiling. The opposite may happen if the light source is not parallel to the walls (windows) but is an uplight. In that case the ceiling should be lighter than the walls.
I don't see anything strange here...
Thanks,
Dionysios -