Yes it is common, CGC grades are very off from anything being sold raw at commic connect, for instance. But that doesn't make CGC wrong, it means the dealer overgraded or missed something. The dealer can say whatever they want. But not CGC, they have to conform to their guidelines across the board, hence the grading notes.
And those help, by the way. Judging from the notes, the Conan book could not get higher after pressing since there is staining, which pressing wont remove. Pressing is mainly good at one big thing, getting rid of non colour breaking creases. there's more of course (like slight folds at corners and light dirt and getting rid of markings if someone wrote on a piece of paper with the comic under it), but that's the one that will have the biggest impact on the book most of the time.
So for your books, it might be good for the Iron man and Submariner book but even then if the spine stress they speak of has colour breaks, then the pressing wont fix that,nor the tanning. It will however fix the light creases. How much would that impact the book, I don't know, we'd have to see it.