Nominal tire width CANNOT be precise because it depends on the rim's width. The identical tire will be wider or narrower depending on the rim it's on.
The actual tire width (approx.) = tire width bead to bead (laid flat) + inside rim width ÷ Pi. Even that isn't perfect, because the amount of rubber on the sidewalls varies, but it will give you pretty consistent comparisons.
Tire manufacturers can't really be blamed here, because they each may pick a different rim to use as the model when sizing their tires. For those who by tires by mail it would be helpful if sellers quoted the bead-bead flat width so you have a true basis of comparison.
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