I can not really reproduce that.    Two different images, same co-ordinates, same (nearly ) Value percentage.
 
   
Color picker or pointer the reported values match.
 
   
I would set the picker radius to 1. Zoom right in. Even then that covers adjacent pixels, see the box, which will average a value. As with most graphics, it depends on the image. Pointer works for your images, use it.
	
	
	
Color picker or pointer the reported values match.
I would set the picker radius to 1. Zoom right in. Even then that covers adjacent pixels, see the box, which will average a value. As with most graphics, it depends on the image. Pointer works for your images, use it.

 

 
