Cornell student here that's taken a bunch of classes at all of the levels - in my experience, undergraduate classes/master's classes are kind of taught with the expectation that you're taking the course as a requirement and are much more evaluation heavy, and that you're likely taking a full course load of other courses as well. PhD level courses are generally assumed to be the only course that you're taking that semester and that you're taking it more because the material will be useful for your research rather than for the credential, so there's much less emphasis on evaluation; i.e. a smaller quantity homeworks that each much harder than an undergraduate version of the same class, and final evaluations are much closer to reading recent research papers and understanding them rather than a timed exam.