Neither can I.
Best I could tell it looked like some tanning in the teal color by Spidey right leg. What he may call spots, could be considered splotches of what looks to be brown or darker teal, like tanning or dirt from handling. Like from handling with dirty hands, which may be a typical place to hold the comic.
@Timed I didn't really "see it either" , but what I mentioned is kind of there on most the copies you posted and or circled?
It's a stretch, but I know cgc ignores manufacturing stuff most of the time, and I would say a consensus is, the way it was printed and whether bleed through from front page and etc.
Could be why you see it on most or what not.