What does the "Reset" button in in Brightness & Contrast settings do?

I have a stack of images with bright spots in every image. Each image has different background variations. If I click on Reset with a different image from the stack every time, it gives me different mean gray values for the same bright spot. I want to understand what “Reset” does to a stack.

Information can be found

https://imagej.nih.gov/ij/docs/guide/146-28.html

https://imagejdocu.tudor.lu/gui/image/adjust#brightnesscontrast

or here in the forum