There are many problems with "mis-using" the focus wheel for range finding. (Don't forget scopes are not designed to range find)
The list of things that can mess up ranging includes;
The way you turn the wheel
Different light levels
Different colour/texture targets
Different temperatures (Of the air/and/or the scope!) The scope may be heating up or cooling down very quickly at times ie. when it goes from house/vehicle to zero range or going from full sun to shade. You may need to give it time to "relax" before setting it up and minimise some of these extremes.
And a problem I'm now working on is eyesight change. I am now sure my eyes focus differently at different times and it may be tiredness or dehydration or age!
Or a combination of all three.
Big Nikko's are known to be good at range finding so you might have an easier time than some people.