Why are the same lessons in different grade levels?

The level of understanding changes even if the topics are the same.

Levels of Understanding Change

The structure and topics will be similar in the different grade levels because the processes, equipment, and regulations are consistent. However, the level of understanding gained through completing the course (and that is required to complete the course) increases in the higher grade levels.

The Associated Boards of Certification has three levels of complexity for their exam questions. Students are required to Recall, Apply, and Analyze information. 

"The following three levels are used to describe the complexity of the questions you will encounter on this exam:

Recall – tasks at this level typically require the simple recall or recognition of specific facts, concepts, processes, or procedures, with little to no problem-solving involved. You may be asked to identify, illustrate, recall, and/or recognize specific information.

Application – tasks at this level will involve some basic problem solving, calculations, or the interpretation and application of data. You may be asked to calculate, categorize, classify, compare, differentiate, explain, specify, translate, and/or apply knowledge.

Analysis – tasks at this level may involve higher level problem solving, evaluation, or the fitting together of a variety of elements into a meaningful whole; they will usually require many steps in the thought process. You may be asked to analyze, evaluate, formulate, generalize, judge, predict, and/or use inductive or deductive reasoning to arrive at a solution."

Source

All Topics are Reviewed

The higher-level courses are also designed to be a review of the preceding levels. Since the exam will likely include questions on things that are outside of daily routines for operators, the basic information is included as a review so there are no gaps in knowledge going into an exam. This method gives the best chance of success for students taking the exams, regardless of level, time in the industry, time since their last exam, or test-taking ability.