The answer is a little from column A, a little from column B.
Printing of these comics was on low quality paper in a cheap way - publishers looking to make some money with disposable entertainment rather than publishing art for the ages. In some comics, the amount tilt between the image and the page its printed on makes it look like someone drunk was involved.
But the scanning (and the person doing the scanning) must also share in the blame. The comics I'm scanning are from a number of private collections donated (sometimes as a bequest from a deceased estate) that are in varying degrees of disrepair. (Some are in really excellent condition, others have been repaired with sticky-tape before donation.) In the library, some can be scanned on equipment available, but some are held in a separate special collection where scanning on a flatbed scanner is not allowed. In these cases - or where the comic is in bad shape that regular scanning might damage them further - I use a scanning app in my smart phone. I'm still learning all the ins and outs, but it scans coloured images better than black and white, and it can over-emphasise variations in the printing, regarding the the variations in the black as intentional shades of grey. I'm getting better with it, but it'd probably be worthwhile in the future to rescan earlier comics to improve unintended quality variations such as you highlight.
There's more to the story, but that gives you an idea of what is happening to cause those quality variations.