Savannah Grace When Lia is weighed, she always formats her weight as 000.00 (example: 101.20). The chapters are also formatted this way (example: 039.00). That's the only reason I can think of.
Nikki It's written in the scale format, but it's how much time has elapsed since she found out about Cassie's death.
Jessica The chapters are, as far as I can remember, listed in order, but they're formatted to match the way numbers appear on an electronic scale. This fits along with Lia's obsession with weight and the constant presence of her eating disorder.