The books of the modern era have different paper and the spine fold has naturally a break along almost all of it. The older books do too often times, but my point is that many "defects" are not damage caused after printing.
Grading weighs all imperfections, and printing defects do receive different weight than other defects. If you read enough, you will find people who think marvel chipping is treated like it isn't there. Some defects are ignored at grades below a certain level.
So, grading is a huge subject of weighing varies imperfections, deciding what each was caused by likely, and how it should affect the grade.
Check out any hard paper stock cover, such as the Platinum Spider-Man. Those covers all have a spine split the entire length of the book. The thickness of the cover paper is the cause, so grading that is judging how much of a split there is? Is it deep and/or wide, or just a narrow slight break of the surface? The handling of those books, and like in the pictures here, how they were packed into boxes, and removed from them, that all adds to the defects.