An American food label can state zero grams of a nutrient while the ingredient list names a source of that nutrient. Both statements are accurate, because declaration follows rounding rules tied to serving size.
Rounding thresholds are written into the rules
Nutrition labeling regulations specify how each declared value is rounded, and for several nutrients amounts below a defined threshold per serving may be declared as zero.
The thresholds exist because analytical methods have limits of precision and because declaring trace amounts to several decimal places would not aid comparison.
The consequence is a step function: a value just under the threshold prints as zero, and a value just above it prints as a real number.
Serving size is the denominator
Every declared amount is per serving, and serving sizes are based on reference amounts customarily consumed rather than chosen freely by the manufacturer.
Still, where a reference amount is small, a package can contain several servings, and the per-serving figure can fall under a rounding threshold while the package total does not.
This is why labels now display a per-package column for certain products, since consumption of the whole container is realistic.
Calorie declarations follow the same logic
Calorie content is itself calculated from measured macronutrient quantities using conversion factors, then rounded to defined increments.
A product with very small amounts of everything can therefore be declared as containing no calories per serving while still contributing something across a day of use.
Non-nutritive sweeteners, cooking sprays and beverage additives are common examples of products where this arises.
The ingredient list is the cross-check
Ingredients are listed in descending order by weight, and that ordering is not subject to rounding. A named source of a nutrient must appear if it is present.
Comparing a zero declaration against the ingredient list is therefore the practical way to distinguish absence from a small amount.
Additives present in very small quantities may be grouped at the end of the list, which reflects their weight rather than their significance.
Why the rules are built this way
Labeling exists to allow comparison between products under a common standard, not to serve as an analytical certificate for a single package.
Uniform rounding means two competing products are described on the same basis, which is the property that makes shelf comparison meaningful.
Anyone whose medical situation requires strict avoidance of a substance, including allergies and clinically managed conditions, needs guidance from a physician or registered dietitian rather than the rounded panel.