Perhaps because this is the subject you have chosen and this is a field you are interested in, you are more motivated and inspired to work harder and engage in the content? With A-levels and GCSEs, you have to follow the learning styles as given to you by your teachers, and you may not like all of the choices of subjects (in terms of GCSEs, there are some compulsory subjects). At university, you are more independent and given more freedom for study. When I started university, the workload seemed quite hard and it was a little difficult to make the jump from A-levels at school to a university style education. However, I was really interested in the subject and got more involved than I did with previous study. It seems like you are doing really well - keep up the good work!