After a student turns in an assignment, they will see a green circle that displays how much of the activity they completed. This is the same circle that will display in the gradebook (as seen below). This circle simply shows how many lines of code they wrote in comparison to the expected solution.
This means that a "not full" circle is not necessarily an indication that the student did something wrong. They may have just found a shorter way to solve the problem than the solution did. For example, in the image below the student wrote 4 lines of code while the written solution had 5.
This also means that sometimes a student will show a score over 100% in the gradebook because they wrote more lines of code than the solutions had, as seen below.
In general, the platform's line counter is meant to be a tool for teachers to use to get a quick at-a-glance look at where each student is in the classwork, and to catch whether students turned in assignments without writing any code of their own. Use it as a directional tool, and aways feel free to dive deeper and open up student assignments!