In my locality, many schools have this tendency to partially include this and that chapter in the syllabus (for almost every subject). For example, (most of the chapters are subdivided in two or more parts), they will take chapter 2, chapter 3.1, chapter 5.1, .2, chapter 14 and make this as a syllabus for a term/semester (out of 15 chapters, say). They complete the full textbook in a whole year, by the way.
Now, the textbook is actually well organised, and a student would understand the topics better if the syllabus followed the order the book is written. The concept of syllabus selection committee is to 'mix hard and easy chapters (parts)' and 'to give different tastes'. In my opinion, the hard parts do not need more work once the prerequisite chapters are done; I mean if you go step by step through the text book, it is always the same amount of difficulty. And for the 'taste', if we keep tasting this and that food frequently, we do not get any taste at all.
So my question is:
Is this normal in other places/countries too? If so, how does this practice benefit the students in learning (mathematics)?