OK, I see 2 issues with the letter. This is just my opinion, I am not a VO, so for what it is worth...
1) The letter says you started as an employee on xx date, and that you are currently in the management trainee program. That suggests that you started in a different (non-trainee) position, possibly unskilled, and did not work the entire year as a management trainee. So the VO has no proof that you actually worked a full year in the "management trainee" position, actually has no idea how long you have done that position.
2) This is more open to interpretation, but based on your duties I would have assigned the NOC 6211. NOC 0621 has higher decision-making authority and functions - a key function is making decisions about money - planning budgets, etc., "determining" merchandise, etc. It is possible you will do those things after you complete the training program, but the description makes it sound like you're not quite there yet. Managers are usually salaried (not hourly wage) employees as well. As I said, this is more open to interpretation and I am not surprised that others have succeeded with the NOC.
I honestly think the first point is the bigger problem.