Here's my goal. In the PDF, all pages are bordered by white blank space, but the white blank space start at different lengths from the border. Scilicet, the cutoff and dimensions of the text and images differ for each page.
Unquestionably it's too inefficient to measure the cutoff of white blank space on each page. So I'm cropping to eliminate the maximum white space, without cropping any of the substantive content.
The important thing about the numbers you type in the Crop dialog is that they are not an amount to cut off the page as you see it, but an amount to cut off the page in its original size. You can see in your GIF the original size (red box), and current crop size (blue box). You in fact set the crop size to the same size it was already cropped, so nothing much happens.