Weird marks on my manga volumes


Completely Average High School Student
I was doing some cleanings in my collection and I noticed some weird marks near the spine on some of my volumes. It looks like that:
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Are these glue marks, or do I have too much moisture/humidity in the house? I noticed it only on a few Gantz and Berserk volumes. Every other manga I checked so far (200+ volumes that were stored in the same place) didn't had any of these. All of my volumes are stored in a bookshelf with doors and I'm worried if I hadn't been taking a good care of it. The inside of it looks fine.


Vampire Ninja
it's from the glue, from what i've heard from people who buy a variety of stuff from dark horse is that their glued books tend to be on the lower end of quality in comparison to other publishers, their stitched binding stuff seems to be fine tho