I mean, it's hard to say, but it could be that the students aren't used to that way of grading. Has the professor used this grading system for very long? It sounds like he is covering the material that will be on the exams. I had teachers that would take off points if the answer wasn't complete enough, or gave incorrect information, which sounds similar and yeah, getting full marks was pretty difficult, but most of the class still passed. If most of the class fails though, that might be a problem.