Looking at the pot of cottage cheese that comprises part of my lunch today, the nutrition information states
per 100g
Protein 11.2g
Carbohydrates 4.3g
Fat 0.4g
Sodium 0.21g
This adds up to nowhere near 100g. What is the remainder? Would it just be water?
Part is water, part is indigestible “fiber” (e.g. cellulose from plants).
In this particular case fibre was listed as 0 grams, but in general if it aint protein carbs or fat it’s water or fibre?
Fiber is a carbohydrate, and is counted as such. The missing mass is water.
It may be technically, in chemical terms, a carbohydrate (cellulose), but very little of it gets digested and metabolized the way other carbohydrates do. It has quite different nutritional properties. I really doubt whether it would be “counted” as a carbohydrate on nutritional labels. (Especially as it seems to have a category of its own on the OP’s label, even though there is none actually there.)
I must say, though, those weights add up to waaaaay less than 100 grams. Is cottage cheese really as watery as that?
How does fiber get into cottage cheese? 
You can doubt it, but it’s true. Pick up any item in your pantry. You’ll find a listing for total carbohydrate and a separate one for dietary fiber.
Well, YOU’RE as watery as that, too. At least half of you is water.
This is a nutritional analysis and the producer has only listed items that they want to show, or are required by law to list. It doesn’t need to add up to 100. A full nutritional analysis could run several pages long and list individual amino acids, salts, minerals, etc.
A proximate analysis would show fat, moisture, dry matter, nitrogen (protein) and ash (what remains after burning) and these would be listed by percent and add up to 100%.
Not to be confused with the ultimate analysis which would show the composition by individual basic elements.
But yes, most of what is missing in your cottage cheese analysis is moisture, and ash.
Cottage cheese is made from milk, which comes from cows, who eat grass, which is a plant.
Yes, but as stated above, the label says there is zero fibre in cottage cheese.
But, at least here in the UK, the “fibre” is not a subset of the carbohydrate, it is entirely separate. Unlike the “saturated fat” figure, which is part of the “fat” total (it goes on to say “of which saturates… Xg” or something similar).
Anyway, yes, low-fat cottage cheese is over 80% water. Linky.
Oh, well, you UK types are apt to do anything. Land of the superfluous “U” and all that. Here in the colonies total carbohydrates are listed, and fiber has a separate listing as as subset of that.
As suggested above, by njtt, the listing for “carbohydrates” in the FDA-mandated nutritional analysis refers to nutritive carbohydrates, i.e., starches and sugars. Dietary fiber (important for good colon function but not a nutrient in the strict sense) would be a distinct figure not included in “carbohydrates”*, despite the fact that celluloses are carbohydrates in the organic-chemistry sense of the term. It’s much like the distinction between the dietary and the chemical calorie.
I’m looking at a box of cereal from the USA here:
So fiber is counted as carbohydrate on USA food labels.
Having looked at a cereal box myself now, this appears to be true, and oddly, apart from fiber and sugars they do not list the other, main carbohydrate component (starches, presumably) separately at all. You are left to infer it by subtracting the amounts of fiber (itself further broken down into soluble and insoluble) and sugar from the total (three boxes I have looked at do not have an “other” category). Frankly, although it is “technically” correct, this is a labeling convention that appears to be deliberately designed to mislead and confuse.