I find Old Navy jeans especially unreliable in terms of size. Once upon a time (at a lower weight) I tried on a size 6 and a size 4, same style...
the 4 was too big and the 6 was too small. No lie. I left the store immediately, and haven't bought pants at Old Navy ever again.
At my lowest weight I was in the 4/5/6/7 range... and yet I bought one skirt from Gap that was a size 1. Ok, maybe I bought it BECAUSE it was a size 1 and fit me, haha.
I even paid regular price for the darn thing.