Why is there sometimes not a 1:1 relationship between figure/ground and positive/negative space?

a.

Postmodernism states all understanding is subjective so as designers we can never know what a viewer will understand as the figure in our compositions.

b.

Because negative space is the darkest area of the composition and black tends to be viewed as the predominant figure more readily than does white.

c.

We can crop the negative space of the composition so that it interrupts the positive space. That breaks up the figure so that it isn't solid anymore.

d.

We can manipulate the composition so that the negative space forms the figure that the viewer understands and the positive space forms the ground.