Prutting my manuscript in book form continues to teach me lessons.
For instance, that line at the bottom of the page on the left begins a paragraph that continues at the top of the following page. In book-design jargon it's called a widow. I suppose that's because it's left alone.
If you see a paragraph at the bottom of a page and its last line carries over to the following page, that line is called an orphan. I suppose that's because it's disconnected from its origin.
Word processing programs like MS Word and Google Docs usually avoid widows and orphans by keeping at least three lines of the paragraph on each page, but book designers do not. I had to teach myself this by looking through books from my shelves and seeing they frequently have widows and orphans.
This explains why some of my pages in the print previewer were one line shorter than the others. When I allowed widows and orphans, those blank spaces at the bottom of the page filled up.
As with most things about book design, I find myself saying, "I never really noticed."
By the way, those clamps holding the book down are from back when I used to repair guitars.