By the skin of one’s teeth

This means that the individual completed a task, but came very close to failing (also far from excelling), i.e. on/around one’s teeth but didn’t reach to an entire group of people, i.e., at a level not too high; by/around one’s teeth) (i.e. at the end), i.e. by the skin of one’s teeth. Jim Needs 100 University Credits out of a possible 200 to graduate, Jim got exactly 00 credits and gets a degree by the skin of his teeth.

Why this example focuses on the fail/success aspect of doing a task, rather than the amount of effort put in.