To not understand size charts

[Songlyrics]

I need to order sports kit for my DC. The company has provided a size chart but it makes no sense to me. It says a 9/10 Yr top is a 34" chest. My 10 y/o DD is average size. Slim but athletic. She wears 11/12 tops normally (outside of school and sports) as she like a baggy fit and I want growing room. Her school games top is a size 30. It's not tight at all but not oversized, either. However, when I measure her chest she is 25.5". Based on the size chart the sports kit company has provided, a 26" top is apparently meant to fit a 3/4 y/o.

Do kids' size charts work differently? Neither my DH or I can understand them. When my DD got fitted for her school uniform last year, the shop assistant couldn't explain it to us either. I am just being stupid or do kids' size charts make no sense?

In the past I have noticed a big difference in sizing for boys and girls, when looking for girls shorts for a 7yr old the measurements were the same as the 5yr old boy shorts.

I'm assuming the sportswear companies sizes are unisex so may be on the larger assumption although 34 inch chest would be about a womens size 10 I think so a little on the big side for 9-10yrs!