I'm shocked that I have cavities?

I'm 16 and I went for a check up and cleaning today, after not having gone for about a year and a half. I found out that I had 7 minor cavities D: I was completely shocked. I take such good care of my teeth. I brush 2x a day, floss, rinse, don't eat sugary foods and... I just don't understand. These are the only cavities I've ever had since having adult teeth. Is this normal?