Shoes vary in size as every company has their own last (the mould around which the shoe is made). Shoe size is also only a function of the linear length of a last and there is no standard for width and shape. There are also a number of different sizing systems (not just by country/region) so not all size 9s are the same, even within the same brand/company as they may use a different last for different models.
Shoe sizes are far from standardised.
Shoe sizes are far from standardised.