Maybe they wanted to get clothes that were slightly looser and more grown-up. Also, quite a lot of tighter fitting things can be worn by people with a smaller size quite comfortably. Primark isn't really the best with its sizing either. I don't think it really matters at the end of the day so long as the clothing isn't causing any harm from being too tight or something. Also, I wear my parent's clothes sometimes and we are not the same size so eh.