Why are lower division science courses so much harder than upper division?

I also found this to ring true with engineering classes as well. Lower division engineering classes seemed more disciplinary, with more homework, and wicked exam curves. Same goes with organic chemistry and other science classes, and the instructors do not easily curve the class, they make it seem as though they are nice. Upper division engineering courses, I could basically relax in the course and it was pretty intuitive what questions would be asked on the tests. With introductory chemistry classes for example, there is such a huge broad range of topics and variation. I can pretty much also take a veterinary pathobiology class too at a very high level, and the course is not hard, whereas the science classes really do take a lot of time at the lower level. I think there is some quota for introductory science teachers grading or else they think just because the grades were low, that people did not know the material because they are young, and thus allow the low grade distribution.