In addition to pursuing "efficiency," Japanese textbook publishers have been creative with the arrangement of math topics to realize "effective" math education programs. What is important here is to make students become cognizant of the connection between what they have learned so far and what they are learning now.
For example, addition with increase in digit is one of the major obstacles for students in the first grade. However, those students, who have already acquired the idea of "ten and some ones" through lessons on composition and decomposition within ten and addition of three whole numbers, will be able to learn and have a firm understanding of the new "make ten" idea. They can solve problems because the newly introduced concept is based on the concepts they have learned previously (i.e., they will soon get used to the procedure to see 7+6 as 7+3+3).
This is why we are always aiming to present new facts and concepts in association with the previously learned concepts.
We carefully examine each topic to see what is common or different, which idea is more general, and so forth. This allows teachers to clarify the points of each lesson and convey them to the students. Meanwhile, students can clearly make sense out of new knowledge and associate them with their own knowledge.