Design is something you learn through experience. And all design projects, even though they may have started from the same assignment, are unique. This means that every student experiences a certain course differently from their colleagues, and that each of them learns different things as a result.
On the one hand, you want to minimize these differences. After all, courses have learning goals, and you want all your students to achieve those same goals. On the other hand, sometimes something happens in a student’s project that gives them—and only them—a valuable opportunity to learn something. As a design teacher or tutor, you should jump on this. You never know when or if another opportunity will arise for that student to learn that particular lesson.
But focusing on one thing means diverting attention from another thing. So by responding to this unexpected opportunity for some unplanned but valuable lesson, you decrease the likelihood of that student learning the thing that you hoped they would learn beforehand. Or at least you might detract from the depth of their learning experience on that point.