I don't know, but it seems like many of these "magazine scans" are actually from an electronic version of the magazine, as opposed to a literal scan of the physical print copy, because on the copies of Megami that I have been able to obtain, all the posters have to be folded to fit in the magazine, leaving a huge crease in the middle.

Also in the cases where I have the hard copy version of the poster, when I compare it side-by-side to the online version of the same image at full resolution, the dots of the CYMK printing process (fairly detectable on the hard copy) don't seem to be visible on the electronic version, even though the pattern appears to be big enough to cause artifacts on a scan. So to make such a clean scan of the print version of the poster without a ton of work, people would need a copy that had not been folded, and need to use some really good image-smoothing processing to clean it up.

I have pondered the same yet the presence of traces of the color halftone after presumably being cleaned would imply otherwise. Also we have to consider that all these files in the pool are not of consistent resolution nor file size what would further conclude that these are in fact very skillful scans.
user:drop in yande.re is just a god but the fact that even they can't get the original colors of the digital image perfectly (post #4613415 vs post #4610871) just shows how imprecise the process of scanning is.