I have a couple books like that as well and was wondering the same thing… man, I wish you hadn’t said ‘stabbed’ instead of slabbed! We might have actually gotten a few straight answers seriously though, I do have a couple books like this and just figured the inner well was a little loose and allowed this to happen. Been keeping an eye on them and no change in the past few months. Still bothers me though!
There were already a few good honest answers above from both Lowgradebronze and Joey. The inner well can cause some of the waviness as well as some of the gloss/quality of some covers. Another issue can be pressing that was improper and later has revision. ( i.e. waviness pressed out but not done properly and later comes back)
Ok. Now that point has been made, knives out.