User blog comment:Paul.rea/Teen Wolf News 081314/@comment-76.20.116.61-20140830213631/@comment-4091815-20140830235645

I actually just went back and watched the Allison scene because this keeps coming up.

I contend her embarrassment at being held back a year was ingrained and had little to do with her turning 17. It was more that her birthday brought up unpleasant questions - not the least of which is the very sexist "did you get pregnant?" - that she had long-since tired of answering.

It was an Allison thing - and was only a big deal to her. Lydia wasn't held back and didn't suffer from the same self-imposed stigma as Allison.

Plot-wise, they needed to make Allison uncomfortable so that Scott could shine with "the understanding" and they would both have an excuse to cut school and bond.

There is also the very real possibility that between writing Lydia's disappearance in 201 (when the Sheriff established her age) and the birthday party in 209 (when Lydia turned 17) the writers forgot that one little scene where being 17 and a freshman was weird for Allison.

I still say in-universe it's not that big of a contradiction because two different characters are allowed to have differing views on the same situation.