arian_arian, I am sorry that you have been rejected. While the job descriptions seem to mostly match what's on the HRSDC website for NOC 4012, I do believe that you may be missing the most important part: the leading statement.
While it is immediately obvious for all of us that you're obviously assisting your supervisor for RA and course instructors for TA, I believe that these things need to be spelled out explicitly so they won't miss the lead statement. For example, on the paragraph where you explained your duties as an RA, you could have maybe written
"As a research assistant, Mr. XXX assists me in conducting research in such and such field. His main duties include conducting literature reviews, surveys, computer simulations and analytical analysis, compiling research results and assisting me in the preparations of journal articles as well as conference papers."
And for teaching assistant, it also needs to be clear who you're assisting. So maybe, you could have written
"As a Teaching Assistant, Mr. XXX assists course instructors in conducting their teaching activities. His main duties include organizing the materials as required by university professors for laboratory sessions, conducting laboratory, sessions to supplement lectures, holding office hours to answer student's questions about the materials of the course, and providing assistance in the preparation and administration of examinations, grading quizzes and laboratory reports, and record keeping"
Of course, you can always try to appeal (several members here were able to overturn the decisions) on the basis that the letter has implicitly mentioned that you have been assisting your supervisor and course instructors in their research and teaching activities, and that your job descriptions match substantially what the website has.
My suggestion for you is obviously try to appeal the decision (since you have some grounds, minimal they may be), and at the same time, prepare a new application.