Light can affect the eye because of a couple of reasons... one, light changes make the pupil open or close, which can affect how you focus your eye ( if you notice a cat before it's about to pounce, it's pupils open right up, because it's using this to create a very shallow depth of field to help it determine the range... like FT shooters do)...
the other problem is that you detect a range by focus, and what tells you focus the best is contrast... funnily enough, because the eye is most sensitive to yellow, the best contrast is between yellow and black, and the eye can be overloaded with the mix of colours that make up white.. but that aside, by lowering the light, you lower the brightest part of the image, which gives you less contrast...which gives you less idea of good focus... which can lead to error.
people rangefind in different ways... and different scopes do... at some point, there is a band of which it's hard or impossible (it varies) to determine a change of focus... fine detail helps determine this better, because it's the first to go in and out of focus, but there's a point/band where your eye my not be able to see this change. Some determine the range on the middle of this band, where they determine absolute focus is, some use the edge of this band, either before or ahead of the range, but the edge of where they can determine no difference... so when the light changes, it might change where this band or edge is determined... if you come up, or come down on the range, it will probably affect if you range short or long on target.
Its personal really to you, your scope, and how you use it (ie what you focus on)... and it may be something you need to learn (if it happens to you) or something you don't. For me, irrespective of what of my scope's i'm using, i use string frays or very fine detail to work off, and the plate is the last resort... and I don't (he says grabbing mahogany) have a problem with dark/light even with the old custom shops. (that said, the custom shops had difficulty on bright white n hazy conditions where i couldn't pic out a sting fray).
I've noticed when comparing scopes, that some handle deep dark colours better at the expense of contrast on brighter subjects, and others the opposite.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep