Well, having some teaching experience, I can say that it just depends on what curriculum the teacher thinks will work best. Coursebooks in general are meant to be treated more like guidelines rather than rigid rulesets that must be followed to the core. For example (since I was an English teacher, I'll be using English courses to illustrate), let's say you're doing Present Continuous as the next grammatical topic. The coursebook might later have a chapter on Past Perfect because those two tenses are similarly constructed, but you think it'd be more useful to move on to type 1 and 2 conditional sentences after that, so that you can introduce all the different uses of the tense to the students so that they can have a more comprehensive view of it before introducing a whole new tense with all its implications.

What I'm trying to say through this is that there is really no one way to organize a curriculum. Textbooks are also written by human beings and every one teacher (textbook authors included) have different ideas on how you can effectively teach the given information. What one person considers a logical order anough to build a coursebook around doesn't have to be logical to the person conducting the course or the students. And since everyone also learns at different rates and with different aspects being important to people, it really is impossible to build one universal curriculum that will not only be easy to teach, but easy to take in as well.

tl;dr: it's all super subjective