I was in the same place as you. My school predicted grades lower than what I was capable of and I managed to prove them wrong and got higher. They said I would never get into uni and yet again I have proved them wrong and now am doing an undergraduate degree in economics.
From what I understood from school, gcse’s are predicated from when you have at your year 6 SATs and tests throughout KS3. In some subjects, end of year 9 tests can determine what grade you are predicted in gcse. But sometimes being a teachers pet can lead you to have a higher predicted grade, which is what happend to some people back when I was at school.
If your predicted grades are lower than what you are capable of, work hard and prove them wrong. Believe me there is no better feeling than proving the school wrong. It is a big f**k you to everyone.