We've all done it.
Do an area render to fix a mistake in your main scene, but hey wait, without changing any settings your area render is not the same as as the original one. Too much contrast, not the same illumination. What's happening here?
Could the ones and zeros be taking crazy pills?
My theory is that Chaos Theory is at work. The initial condition should be the same in both renders, but the calculation in your PC must be within a dynamical system, where a little miscalculation leads to a butterfly effect.
If that is the case, no two renders are never, ever the same.
Or maybe I've been taking too many crazy pills.

Crazy pills.

My area renders match up without fail.  Poser is a mess, but not that dynamic as to suffer major butterfly effects. If to bizarre, just close and re-open the scene. It should always be the same, reading from the original file.  Just save before rendering.

I cannot save the world. Only my little piece of it. If we all act together, we can save the world.--Nelson Mandela
An  inconsistent hobgoblin is the fool of little minds
Taking "Just do it" to a whole new level!