One thing very few people understand, even engineers, is that EVERYTHING has variation in its characteristics. Whether it is a dimension (size) or a mass or,... whatever. It is not a fixed value. Typically characteristics are given as "nominal" values (average/mean, median, typical, target, ...).

Even then, the actual tire width can be dependent upon the rims they are on or the pressure inside. How would you like to decide whether to use this tire or that one when the sizes are quoted at two different pressures mounted on two different rims?