I don't know how to describe it, but I can tell you why it is. Please excuse me if you already know this stuff.
They take a very large piece of paper, fold it into how many sections they want, sew the spine, and cut the other edges so you have separate pages. Each of the "sections" you are talking about is one sheet of paper. Old book sizes were based on how many times the paper was folded, like folio (folded once into two sheets) or octavo (8 sheets, I think, and abbreviated as 8vo, again I think, it's been awhile.)
Also in older books, they would cut the top and bottom but leave the side together, so the first time you read the book you have to cut the pages with a penknife. You have to cut a section every 8 or 12 pages or so. This makes some edges longer than others. In the States, for a first edition or some other nice hardback, they will leave the pages uneven to mimic this look.
You can find references to this all the time, if you read any Shakespearian criticism, they will reference his early published collections by their size, i.e. 1st folio. Also, in The Great Gatsby, someone is commenting about the size of the collection in his library, and says, "He didn't go too far. The pages aren't cut." implying that they've never been read.
Sorry to be all professorial, but I find all this stuff fascinating, the creation and storage of books.
If you are interested, check out "The Book on the Bookshelf" by Henry Petroski. Not so much about how books are made, but how they are stored. It's really more fascinating than you'd think, how we arrived at the current system over hundreds of years.