How does imagej measure pixels in circular ROI?

I am trying to measure photon counts from single molecules (7*7 pixels) by
defining a ROI around the single molecule. I localize the center of the
emitter (this could be subpixel) from ThunderSTORM and draw the ROI around
it which makes everything sub-pixel. If the ROI is circular, some of the
pixels are excluded from measurement. Can someone tell me how exactly the
pixels for measurement are selected? Is it based on the amount of area
selected by the ROI boundaries or is it something else?

In the above images, I specified a sub-pixel center and drew a circular ROI
and filled it from the ROI Manager, but the fill seems to fall completely at
a different location than the ROI specified. I am not able to understand how
this works! Please help.

Crosspost of:

http://imagej.1557.x6.nabble.com/how-does-ImageJ-measure-circular-ROI-td5022481.html

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