I wouldn't say it's a well-known fact that upper-division courses are easier. I would say that since most people take gen bio and gen chem their first or second year that they have a harder time initially, but eventually learn to develop better study methods and know their limits/strengths on the material they are learning. Also, to the OP, upward trend does play a significant factor. I averaged about a 3.0 gpa overall and science for my first two years in undergrad, and then I got 3.7, 3.8, 3.9, 3.7, 3.65 for my last semester. I would have gotten a 4.0 if I didn't have to take biochemistry, but luckily I was able to pull off a B-. The point I am trying to make is that upper division courses are going to be more detailed and difficult since it's not touching the surface anymore. However, it is easier to some degree since most people study harder for these classes since their relevance in dental school is significantly higher. I don't think UCSF (my #1 choice school, which I am attending this fall) would have even considered me for an interview if I didn't have an improving GPA.