Interesting post. I've noticed similar results myself.
When I shoot indoors, I always place a white or chartreuse 2" square in the center of an NRA 25 yard slow fire target. This target has a 5" black center area.
My groups are always better when I have a high contrast and small aiming point.
I've never tried reversing the contrast like you did, but I wouldn't be supprised if my results were the same.