Grain Focusing - discrepancies
Did you happen to follow the manufacturer's instructions on setting the focus of those grain finders/focusers? Also did you place a sheet of paper under the finders the same thickness of the paper you are printing with? Another issue can also be vibration after focusing.
I have used a number of finders in the past, my favorites are the 25x microsight and my peak 1 focuser. I have also used the hocus focus sometimes which works fairly well with such a simple design. The larger peak helps with viewing corners. It also has a blue filter which has been debated about its effectiveness.