Difficulty levels are more of an art than a science and differ from person to person or even from one time to another time for a given person. So I wouldn’t sweat the precise difficulty too much as long as you fall within the right “displayed” difficulty on the website. After all, that’s what the student sees (easy, medium, or hard) with no way to distinguish between an exercise marked level 2 or one marked level 3.
I believe it’s 1 - 3 for easy, 4 - 6 for medium, and 7 - 10 for hard,