Do these measurements seem off?

I wanted to buy this outfit for a bit of a gender reveal we're planning. I thought it would be best to measure Ivy because right now 9-12m is too small but 12-18m is too big. I measured her bust (42cm) and her tshirt length (23cm) and according to this, even 6-12m is too big for her?

I mean I know she's small for her age but... that seems insane 🙈

I mean, this tshirt looks like the same material as her "big sister" tshirt we bought last month, which was 0-12m and was huge on her 😅 so maybe UK sizes are just smaller?