Though the length of the foot is used for determining the shoe size, there are a lot of variations in the foot of individuals with the same foot length. The width of the foot, angle or slope of the foot will vary due to a number of reasons like hereditary factors, weight, health, lifestyle, gender, race, climatic conditions. Most shoes and footwear are designed for people with narrow feet and individuals with larger/wider feet often have to purchase shoes of a larger size.
Compared to other footwear like flip flops, slippers, the shoe designer has to be more careful, as the shoe has to be closed or there is some way to ensure that the shoe does not come off easily. As the factors involved in the design increases, the possibility of an error or mismatch will be more.