My screenshots are variable sizes while using the same selection rectangle on the same display

I am running Sonoma 14.2.1 on a MacBook Pro 16-inch 2021. I have an attached Arzopa screen but everything I'm describing occurs on my main laptop screen.

Without any discernible pattern, some of my screenshots are 2954x1926 and others are 1477x963. I'm using the same selection rectangle in both cases. In most cases, I don't even move the rectangle on the screen - I just invoke ScreenShot and then click the "Capture" button.

I always want the larger images when I do a screenshot, as I usually want to easily read what is on the screen I capture. I can't seem to stop the system switching back to capturing 1477x963 images. Of course, I can't tell when it takes the screenshot what the resolution it used to store the image and by the time I find out, the Zoom call or whatever has moved on and I can't reshoot the screen capture.

How do I ensure that every screen shot will be stored with the maximum image resolution?

Details - using identify on the most recent unmodified two screenshots with identical, unmoved selection rectangles, I see this:

% identify Screenshot\ 2024-01-30\ at\ 20.59.17.png 

Screenshot 2024-01-30 at 20.59.17.png PNG 2954x1926 2954x1926+0+0 8-bit sRGB 985989B 0.000u 0:00.003

% identify Screenshot\ 2024-01-30\ at\ 20.37.39.png 

Screenshot 2024-01-30 at 20.37.39.png PNG 1477x963 1477x963+0+0 8-bit sRGB 305341B 0.000u 0:00.001