A nutrition panel is a reliable statement of what a food contains. It is a poor guide to how that food will behave, and the gap is structural rather than a labelling failure.
The panel measures composition, not structure
Laboratory analysis breaks a sample down and reports the quantities found, which produces accurate figures for protein, fat, carbohydrate and salt.
Breaking the sample down also destroys the thing that determines digestion speed: the physical arrangement of those components in the food.
Two foods with matching panels can therefore be digested at very different rates, because one is an intact cellular structure and the other is a powder reassembled.
Serving sizes are chosen, not measured
Figures are reported per serving as well as per hundred grams, and the serving size is a manufacturer's declaration rather than an observation of what people eat.
A pack that plausibly contains one portion may be labelled as two, which halves every number on the front without changing anything inside.
Comparing the per hundred gram column avoids this entirely, which is why that column exists even though the serving column is more prominent.
Ingredient order carries information the panel does not
Ingredients are listed by weight, so position indicates proportion even where no percentage is given.
Related ingredients can appear separately under different names, which places each one lower in the list than their combined weight would justify.
Reading the list alongside the panel catches this, since the panel totals what the list divides.
Nothing on the pack describes the meal
Foods are rarely eaten alone, and the presence of fat, fibre, acid and protein alongside a food changes how quickly it is absorbed.
A panel describes an item in isolation, so it cannot express that the same food eaten with a mixed meal behaves differently from the same food eaten alone.
This is the main reason single-food judgements made from panels tend to be misleading in practice.
Claims on the front follow different rules
Front-of-pack wording is regulated but generally describes one attribute, and a food can qualify for a claim about one nutrient while being unremarkable in every other respect.
The claim is therefore a statement about a threshold rather than an overall assessment, and it is written to be legally defensible rather than informative.
Anyone eating to a clinical requirement should have the panel interpreted by a professional who knows the requirement, since thresholds and individual needs rarely align.