I think it differs for everyone. Certainly at my university the full range of marks are used in marking an exam, so people get everything from fails to firsts. I believe most people fall between 50-75% though.
It's hard for me to comment year on year, as my second year exams were a very different format to most of my first year exams. In my first year, the majority of exams were testing knowledge, so mostly short answer questions and multiple choice, sometimes with one essay to write as well. We also had an exam which tested our skills in interpreting scientific papers. In second year, all my exams were entirely essay based, having to write 3-4 essays over two hours. I think it's much easier to get very high marks on the type of exams I had in first year. I was happy with the marks I recieved in second year, but I think writing a timed essay is much harder to get very high marks from.
So for my course many people's marks decreased in second year. I imagine in a course where exam styles were the same each year, some people would increase their marks (through improving exam technique/motivation/knowledge) and some people's would decrease (due to content being harder/trickier concepts to understand/a higher level of work being expected).
I know you didn't ask about coursework, but I felt coursework was more harshly marked in second year, so the same might be true of exams.