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The question remains:  What is the histogram measuring when it gives me "Pixels: 103471" in this picture, yet the measurement log gives the correct area of 6621510 for the elliptical marquee selection.

I've completely reset Photoshop to default settings/preferences

My goal is to calculate the percent of green pixels within the selected area.  I've been using the number which I've highlighted for you in the attached image below as the divisor.  Then after selecting the color range within the selected area, I use the new number in the histogram next to "Pixels:" as the dividend.  So in the examples shown the pictures, I would do 1247/103471 = 0.012051.  Should that result be correct?  It seems I should be dividing into the Area, not whatever number the histogram is giving me. 

Why would the histogram show the correct area for a 100x100 circle, but not for a larger selection?

It looks like the histogram stops showing the correct area once you start drawing circles that are larger than a certain size.  The histogram showed the correct area for a 600x600 circle, butnot for a 700x700 circle.

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