You definitely asked a question, you just left out the question mark.
I will attempt to answer it. Because circles in sketchup are not true circles but are actually polygons, the flat sides will measure smaller than the precise radius, which is found when measuring at the apex points of the polygon. Sketchup is not doing anything to your model you aren’t telling it to do. If you measure your circles from the center to a point on the radius, it will be “dead on,” guaranteed.