Each time when you review a line, if you are able to solve the moves correctly, your level goes up. A difficult move stops being considered difficult once you have gotten level 4 (+70 points) in a review for it. Until then, it will keep showing up as part of your difficult moves. Getting it right during an overstudy session doesn't count.

To learn what a difficult move is, please see this page: What are difficult moves? And how can they help me?