I'm not an English speaking person, but while I agree with you on what an album is, I cannot agree on gallery.
In order to understand the logic we should first understand what is an album and what is a gallery. And album is easy. Probably, everybody owns a photoalbum, which holds family photos and other such stuff. Totally agree with you on album being a collection of objects (in our case, pictures).
Now gallery. When an average person hears the word "gallery", the first thing that comes to their mind is an art gallery. As in, a room or a building for exhibiting pieces of art.
So. Books will fit into buildings. Buildings cannot fit into books. :) Therefore, among these three types of objects gallery is the biggest and the most general and should be considered top-level: gallery holds albums/volumes, which, in their turn, hold objects. This is logical (and supported by dictionaries).
However, I cannot decipher the logic behind galleries being chapters, as you say. I don't understand what you (or the plugin developer) see in your mind's eye, when you hear the word "gallery". Does the word mean something else entirely in your language?