I always thought that "Schwund" was the tail wiggling the dog - the outer pages are too short, not the inner pages too long.

Anyhow, here's a quote from the link below:

Figure 3.20 Paper thickness causes edges of pages to creep outward during binding. When the finished pages are trimmed, artwork on the inner pages will be closer to the trim edge.

...

The fix? To maintain a consistent outer margin despite the page creep, the page content must be shifted incrementally to compensate, a process known as shingling. The closer a page is to the center of the magazine, the more content must be moved very slightly inward.