Baby Gap sizing mystery

Aliza needs a 12-18 in Baby Gap pants. (My mother sent me 2 pairs of their jeans--I think I won't be getting them again as they're cut quite tight in the crotch; even over disposables it's tight, much less cloth.) I have both 6-12 and 12-18 in their jeans and I'm sure she needs the bigger size--they're too short and barely snap at the waist, and Aliza's not chunky in the middle.

However, she has Baby Gap 6-12 tops that are huge. Makes no sense.