VPhot Effect of Signal Circle Radius graph

Hi,

I am wondering as the meaning Effect of Signal Circle Radius in the Measurement Details page in VPhot. Please see attached for a screen capture for my example.

I take the blue curve to be the SNR, with the SNR of 205 corresponding to my aperture radius of 7 pixels.

However, in the black curve, the Instrumental Magnitude of -7.653 lies outside the range of the y-axis values of from -9.0 to -12.0.

I wonder would it be possible to label each curve.

John