The numbers on a food label are accurate, and they routinely mislead. The reason is that almost every figure depends on a serving size, and serving sizes are defined by rules rather than by behaviour.
A serving is a reference amount, not a recommendation
Regulators define reference amounts for food categories so that products within a category can be compared against a common basis.
The reference is intended as a measuring convention, in the same way that price per unit weight allows comparison between package sizes.
It is frequently read as guidance on how much to eat, which is not what it was designed to communicate and not how it is derived.
The reference is anchored to typical consumption, imperfectly
Reference amounts are generally based on surveys of how much of a food people report eating, which means they lag behind changes in portion norms.
They are also averaged across a whole population, so they describe nobody in particular and sit below what many adults actually consume.
Because the figures feed into legally defined labelling, updating them is slow, and a reference amount can remain in force long after habits have shifted.
Package size and serving size are separate decisions
Manufacturers choose package sizes, and packages containing several servings are common in formats that most people finish in one sitting.
The per-serving column is then correct while describing a portion that few purchasers use, and the total in the package requires multiplication to recover.
Rules in some jurisdictions now require a second column showing the whole container for exactly this reason, though coverage differs by country.
Per-hundred figures allow real comparison
Values expressed per hundred grams or millilitres are unaffected by serving decisions, which makes them the only part of a label directly comparable between products.
Comparing two items by their per-serving figures can reverse the ranking entirely if their declared servings differ in size.
Reading the per-hundred column first and the serving column second removes most of the confusion that labels generate.
Rounding and tolerances are built into the numbers
Labelling rules permit rounding and allow a margin between the declared value and the analysed one, since natural products vary between batches.
Small declared amounts can therefore appear as zero when a portion contains a little, and this becomes noticeable when several servings are eaten together.
Requirements differ substantially between countries and are revised periodically, so the conventions on a label depend on where the product was intended to be sold.