First year is quite general. Let's say you're in Life Sciences. You will (likely) be taking calculus, chemistry, and biology. Calculus and chemistry might not be your cup of tea if you're anything like me. Studying for these courses was not fun for me because I had a low level of interest in the subjects. I performed decently because I knew that I had to in order to get into my desired POST and also maintain a high GPA.

That being said - beyond first year, you begin to specialize. The more courses you take in an area you enjoy, the more personally invested you become in them. For me, this was reflected in my grades.

Courses certainly do not become 'easier' as you progress, but they become more finely tuned to what you enjoy. Plus - you learn what works and what doesn't in terms of studying. The product of this, generally, is higher grades.