When modifing the selection in a Photoshop layer mask, the modifed region has a different density th

Thank you for your reply.  Here’s the image in Photoshop 26.1.0  (Beta) after I’ve applied some simple contrast, highlights, and shadows adjustments with Camera Raw to Layer 0 configured as a Smart Object.

What I then wanted to do was enhance the contrast in the sky to make it look more dramatic.  I then was use Photoshop to Select > Sky in the hopes it would be quicker than trying to use another tool to get close to what I wanted to select for a mask on a Brightness/Contrast or Curves layer.

I assumed that the “marching ants” defined the area that Photoshop considered to be the sky and that everything within the "marching ants" was fully selected.  More on this later.  There are a couple problems with the selection.  I realize that this image presents some challenges for an automated tool because there is little differentiation in color or brightness between the pre-sunrise sky and low hanging clouds, fog, and even a couple mountain tops.  In the middle of the image there is a small portion of the sky that was not selected automatically.  I assume it was because of the presence of fog and mist.  I wanted to add that missing area to the selection.  There were also section of the clouds and mountain tops that were selected when I didn’t want them selected.  Later on I will delete those areas from the selection, but that is not part of the issues I’m trying to understand and resolve.  In the following image I’ve zoomed in and used the Polygon Lasso Tool to add to the selection between the two mountain peaks to add some sky that was missed.

The selection above has the “marching ants” where I expected them to be in this part of the image.  In the following image I’ve applied the mask to a Brightness/Contrast layer and for illustrative purposes decreased the brightness substantially.  The area that I added with the Polygon Lasso Tool stands out as being much darker although in the original image the brightness is a gradual, smooth gradient from one area to another.

Now doing the Alt + Click on the layer mask shows this:

I learned something here.  It would be a good idea to do the "Alt + Click" on a fairly routine basis just to make sure that what is actually selected for the mask is what I intended.  The area I added to the mask clearly stands out.  It is white while the selected sky is a light gray.  The added selection looks like a setting sun in the middle of the image above.  The next thing I notice is that there are areas outside the “marching ants” boundary of the mask that are selected (they are not totally black), likely partially selected because they are low clouds and of the same color and brightness as some of the "sky", but they are in front of the mountain peaks.   I’m no expert but, it looks like the Select > Sky operation did not fully select all pixels in the main part of the sky.  It seems to have been fooled by the variation in gray tones. To the contrary, the Polygon Selection Tool seems to have fully selected all pixel within its selection boundary and added them to the existing selection.  When the Brightness/Contrast layer was adjusted it had greater effect on the portion of the mask created by adding to the original Sky selection.  I’ve often used the sky selection tool to apply adjustment layers to the sky, but that was mostly done with blue skies and possibly some white clouds with nothing in common with the non-sky portion of the image.  This image is different from others where I’ve had good success with Select > Sky.  I've done a little bit of web searching to see if there are any identified limitations of Select>Sky but so far I have not found any posts.

Thanks in advance for any further insight you might be able to provide.