Young feet contain soft tissue between the developing bones which does not completely harden until mid-teens. This means that the size or width may vary with small movements of the foot or the angle of the leg, even if the child is relaxed.
The measurement given in store is a starting point. Once your child has a pair of shoes on their feet, the fitter may size up or down to find the best fit.
We do fit styles individually, rather than giving a blanket size. This is because individual styles may fit differently, especially in the case of sandals vs canvas shoes for example.