Why are shoes measured with Plus12?

Because Plus12 is a measuring tool that shows the actual space inside the shoe that the foot can use. Insoles are generally always shorter than the actual usable space inside the shoe. Therefore, it is not entirely correct to say that the shoe is small when stepping on the insole. It gives some idea, but take into account that there is actually more space inside the shoe.
In the picture, the inside length of the shoe is 17 cm, but the insole length (stp) is 5 mm less (16,5 cm). Therefore, it is not possible to get the same result when measuring with a tape measure at home as when measured with a Plus12 in the store.

The picture shows bLIFESTYLE Gibbon children's winter barefoot shoes with a wool lining.

Read more about measuring with Plus12 from here.