Your eye sees the entire picture and judges contrast, the densitometer sees one step at a time and measures contrast. These two pieces of data do not need to correspond. Thus, fog is different to you as the observer and the curve that might be drawn.
In making enlargements or contact prints, fog tends to lower contrast even though it is uniform throughout. That is because it lowers the difference between Dmax and Dmin. However, by use of the right print contrast, this can be removed.