This is definitely a reproducible thing. I had no idea it was happening before I read this thread.
I just tested it out on a junk slab. Originally it looked very nice with no vertical scratches. Then I put it in one of the graded card (PSA-size) sleeves. One of these:
[Look at some of the 1-star reviews on that page. You can see people talking about the same thing. One person included an example picture.]
After testing it on a slab I noticed that tons of vertical scratches appeared on both the front and back, even though I only put it in and took it out once. By design these are tight fitting sleeves, so it’s natural for one to want to apply some pressure on the face of the slab as you slide it into the sleeve.
If you look in the right light with the right angle, the scratches are super obvious. However, if held in a normal position, not an angle, and in normal lighting, you might not notice the scratches at all.
As mrtz mentioned, hand placement really does seem to be key to mitigating this issue.
It’s really annoying that the PSA slabs scratch so easily. Now I wonder if there are other sleeves that would be less likely to scratch, while still being just as tight fitting…
Many more examples can be seen here:
https://www.blowoutforums.com/showthread.php?t=1522481