too late in eve to go through, but it's a subject i believe can be explained a bit better when i have time...
to do with depth of field, reduced light, scope's focus capability and the eye's capability to see that focus, and wether you call the range when you see the focus get sharpest, or wether you call it when it's in the middle even though the focus may seem the same for a yd or so...
in short, a few things may come into play... in addition, the eye determines focus by contrast, and if you change the light levels you raise or lower the contrast, making it easier or harder with all the above involved.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep