I would imagine there is a lot more to it than most folks consider, perhaps even the 'modern' book printer (without the previous generations experience fully shared).
Consider this; paper doesn't come in the size of the book, or even a cut sown sheet, or even larger sheets (say the size of a pool table), it comes in rolls possibly 8-24 feet (making up numbers) long and 6-8 feet in diameter. When they feed it --onto the roll-- I suspect the paper is dry, flat, and the same size from one side to the other. As it wraps onto the roll, extremely fast, maybe hundreds-1,000 ft/min., the outside must stretch to match the increased radius determined by the paper's thickness--meaning, card-stock requires more stretch--with naturally that amount of stretch decreasing as the roll gets larger (smaller proportion of difference to length). Same-same with steel.
They'll slit the rolls to shorter, more manageable lengths, and gradually cut it down in smaller length/widths--but, as they pull it off the roll((s) as they may re-roll it a couple times before flattened), that paper being longer on one side or the other will curl--with the greatest curl near the roll's center. SO, part of the issue
might be what part of the roll your batch was run from. However, at the point in the process where they are making sheets, no matter what size, they must flatten the paper.
In steel they call it 'leveling,' sometimes stretching, yet in a nutshell they very aggressively bend/flex it one way, then back, and also along its axis lengthwise. It's similar to how you unroll a rug and must then re-roll it inside out to get it to flatten. However, with paper/steel, all that bending takes place over maybe 12 feet as it feeds off at 1,000 feet/min.. With steel it's mechanical, with paper--I'm guessing--they could use heat and steam along with the mechanical aspect. Essentially they are stretching the short side to match the long side as best as possible. What that also means is, which side of the paper is 'out' on your book may determine the risk of curl as seen in
@Astro Pen 's books.
Now add the variable of different thermal/humidity expansions of the paper to outer coating, and you can see where if it came from the inner roll, they didn't stretch it properly, and didn't use the correct side, it just compounds the condition caused by the coating. And that may explain why some books (from different paper stock runs) curl more than others.
K2