You are very observant. But sort of mistaken. Let me explain.
Cammie writes each book (or each book so far) during the school break that follows the semester in question.
So Cammie, the narrator, is telling the story of LYKY during winter break–in other words during late December or early January. So when she says “I’ll be sixteen next month” she’s talking about turning sixteen in late January or early February (to tell you the truth, I’ve never really narrowed it down.)
So she actually turned 16 during the course of CMH. Initially, there was a scene in there about her birthday, but it didn’t serve any purpose, so it got cut somewhere along the line.
Posted in: Gallagher Girls