GCSE and A-level study practices made students stupid?

I was never the best student, but during my GCSEs, a friend who excelled in exams shared his secret with me: practice past papers relentlessly. Before this, I was an average student, scoring around 5-6, not for lack of effort, but because I studied as if I were in university, trying to deeply understand the syllabus. Once I started focusing on past papers, my scores improved significantly. I continued this strategy through my A-levels, and it worked well.

However, university was a different story. I actually had to learn the material, which felt frustrating. Despite this, I managed to get an 8 in GCSE Biology and a B in A-level Biology. Ironically, I ended up studying Computer Science at university, a subject I had no prior knowledge of, yet I performed better. Interestingly, many people who struggled with their A-levels and GCSEs actually did much better at university. If you asked me anything about biology today, I wouldn't know how to answer. This experience made me realize how flawed exams can be.