Accurate count of samples passed

I am implementing a lens flare effect that uses an occlusion query to tell how much of light source if visible. The more pixels are visible, the brighter the lens flare.

I’m having trouble getting an accurate estimate of the maximum number of pixels that could be drawn. The primitive is a GL_POINT with a point size of 16. My samples count ranges from 0 up to about 806. This confuses me, because at most I would expect a 16x16 point drawn to cover 256 pixels. Any idea what’s going on here?