The narrator defines bad behaviour as either mischief or mistake:
- Mischief is when you did something you knew was wrong.
- Mistake is when you did something that you didn't know was wrong.
Using those two buckets, the narrator categorises their own behaviour: "I didn't do mischief, but I did make mistakes."
(The narrator then expands with a story to explain what happened and why they make that judgment, which you may or may not agree with.)
So when we are categorising things, we can say they are X and/or they are not Y.
Why is this important to the narrator?
We make this kind of distinctions when we want to be clear and we don't want people to misunderstand.
The narrator in this snippet (the football player, Adam Goodes) wants people to think he was a good kid, just a little silly at times.