why mix a confession and an excuse

I'm confused about the premise for the whole book. If he is writing his confessions to god, why make excuses? A confession of guilt from someone claiming to not be guilty doesn't seem to make sense. What he did when he was a child wasn't his fault. What he did in a group wasn't his fault. Where is the true confession here?

stealing pears

I was looking up things about the confessions to see if I could get some insight to help with the reading and I found an explanation about when Augustine stole pears only because he could share in sin with others, and this was a basis for group mentality. I thought it was interesting to have that as a foundation for group mentality.