Further to Stallzer's post.
The hinge when removed incorrectly will take the gum, and, part of the stamp paper as well, thus the paper is "thinned" and is seen held to the light, or under UV light.
Coincidentally, the one of the very reasons hinges came into being, not only to affix the stamp to the album page, but to hinge the stamp on an axis, so collectors could look at the back of the stamp without removal from the page.
'Hinge "thins" are considered
If you hold your examples with tongs, up against a light source, you should be able to see the "thin" yourself, from the front of the stamp.