I will stand to be corrected, but every scope I have had including Leupolds, Burris and my Sightron, all do this, or should I say "my eyes do".
I think what might change, is how much difference there is in different lights with different types of quality of glass/scope.
If I try to in read in bad light, I need my reading glasses, if I am in good light I can usually get away without. I thought that this was a similar issue, and put it down to my eyes not receiving enough light in order to focus in the same way.
I would be interested to see what others think.