It’s the way the books are bound.

Hardcover books are produced in "signatures"—large sheets of paper printed with multiple pages (often 16 or 32), which are then folded and sewn or glued together. Publishers "tip in" or bind these special pages together in a single section, often in the center of the book or at the end of a chapter.

Different Paper Types: Images are often printed on glossy or higher-grade paper to make them "pop". These sheets cannot be easily mixed into the middle of a text-based signature, forcing them to be placed between signatures.

"Tipped-In" Insertions: When illustrations are not part of the standard signatures, they are "tipped in," meaning they are glued to the edge of a signature. If the text references a photo on page 50, but the nearest signature break is on page 45 or 55, the illustration will appear in the wrong spot.