Help Measure - How Works

Hello,
With this image, I would like to understand how the “Mean” calculation is done, the steps I took were:

  1. I opened a DICOM image
  2. I made a circle that is in yellow in print
  3. Click on the menu “Analyze” -> “Measure”

Capturar

Does it use multiple layers to calc or something else? because in my test of always 255.