Architectura Hydraulica can generally be divided in two sections, the first one being a list with explanations, and the second being the illustrations that each explanation corresponds to. This specific composition causes different cases of content distribution on the pages. The spreads selected here are meant to showcase this peculiarity of the book.
The first spread represents the typical layout in the list section of Architectura Hydraulica. Therefore, both pages are merely covered in text, which is evidently structured into multiple list items.
The second spread marks the transition into the illustrational section of the book. While page 12 still shows text, specifically an instruction to the bookbinder, page 13 already shows an illustration.
The third spread demonstrates the way the illustrations are incorporated. The left page is empty, while the right one folds open to uncover the illustration. Presumably, this arrangement was chosen to protect the illustrations and account for the larger size of illustration shown on this last spread.