What is the difference between the respective fault of a contractor and a plaintiff who failed to wear a seat belt?

In Heller v. Martens, the court was assessing the respective fault of a negligent driver and an injured plaintiff who failed to wear a seat belt. The fault in each case was negligence. In this case, the fault of the defendant lies in a breach of an implied term for the performance of a contract for services for the construction of works and the fault of the plaintiff lies in failing to take reasonable care and make reasonable inquiries as to the experience and competence of a person hired to construct the works in question. In my view the fault of the person who does the faulty work is greater than the fault of the person who hired the worker. The defendant had or ought to have had greater knowledge than the plaintiff as to what was required to construct the works in a good and workmanlike manner. The defendant was present throughout all of the works and observed or ought to have observed the many areas of very sloppy work where ridges were left in the concrete surface, where unevenness was evident, where forms had bulged resulting in curved edges where straight edges were obviously intended, where steps were uneven, and where the slope where the driveway met the street was such that it could not be negotiated by passenger cars. The defendant knew or ought to have known that the placing and finishing of concrete required skills which the defendant did not have.