RED epic loss of information, why!

Recently, when shooting a well lit, stable product, at an ISO of 640 and at 200fps we were getting discolouring and noise appearing in the dark areas of the scene, there appeared to be no real reason for this, as the main point we were focusing on was well lit and perfectly exposed.
When exposing this more for the background, it of course got rid of this, but why would it become noisy when the point of exposure was perfectly exposed?

Help creative cow users!