If I remember correctly from my early purchasing days, it is related to single sheet printing/folding/cutting. A single sheet of paper folded twice ends up having 8 faces; folded 3x yields 16 faces. The standard book size is set so that standard printing presses can generate 16 pages per print. This makes 64 a magical number as it requires 4 print sheets. The 88 page is probably from a 2-fold printing process yielding 8 pages per print. There are different size presses and paper stock. Then there is the binding, which is lots of different formats and processes - but effectively if your book it too big or too small, you lose some of the options because they simply won't work. I am sure there are a lot of others in our membership who can speak with greater expertise than this - but for the basics this is the way I remember it.