Interpretation of Google Analytics screen resolution data

I'm trying to optimize my website in consideration of the screen resolutions most commonly used by visitors.  However, my Google Analytics data seems to say that the greatest proportion of visitors have an unknown screen resolution (screenshot embedded).  How can I interpret this?
Screen-resolutions.jpg
ddantesAsked:
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Dave BaldwinConnect With a Mentor Fixer of ProblemsCommented:
That looks a lot like the display on my account.  It is possible to block reporting of display size and characteristics and that may be what we are seeing.  I would suggest you ignore it as unknown and unknowable.
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