Hi,
Okay so I'm also a Year 11 doing Biology 3 & 4... but I didn't do 1 & 2.
Just because you don't do as well in Unit 1/2 does not necessarily mean you will do badly in 3/4 - because as long as you are really enthusiastic about the subject and consistently work well, you are setting yourself up for success!
When I was reading through the ATAR notes forum, a common theme is that they work even harder after getting a disappointing score. Remember that Year 11 is like an experimental year - this is the time where you should be testing out study techniques and if you don't get the grade you want, you adjust accordingly to meet your goals! It's really great that your post reflects that, you grades are consistent but maybe it's just that burden of doing well in Biology and your other subjects that hindered your current exam score? So maybe ask your teacher for some more exams and practise distressing techniques so you have a clear mind during exams and maximise your marks
Btw it's kind of a good sign that you're disappointed in your score...because that means you have high standards for yourself and strive to do well. It's just that what you do after you receive your grade that counts.