You've answered your own question.
Go over what you got wrong in all of the completed papers and improve on them, If you're getting top marks it will be easy to see where your skills lack, even if its dropping 1 mark on a 5marker. You need to see what you can improve on, that might be using context more, developing your answers, being more accurate on your multiplication or even using the equations.
You should be happy with an A, it could be much worse and at the end of day - if you get an A in the exam, you can hardly say that you failed. You have access to pretty much every course that requires X grade in Science.