One of the questions for my lab is to find the non-fiber, non-sugar carbohydrate per serving of an onion. Now the total carbohydrate for this onion is 11 g, the total sugars for the onion is 9 g, and the total fiber is 3 g.
The individual values in the OP's onion label are not out of line with the U.S. Department of Agriculture's reference data. There are two of the latter, one for "Onions, raw" and one for "Onions, sweet, raw". Their figures would lead to expecting carbohydrates of 13.8g or 11.2g, respectively, for a 148g sample.
The negative difference can be explained by simple rounding error. If the measured carbs, fiber, and sugar values were actually 11.49, 2.51, and 8.51 g then the label values would be rounded to those shown on the label, yet the difference of the measured values would be a positive value: 0.47 g.
The values on the nutrition label from the question (11 g total carbs, 9 g sugars, 3 g fiber) could be actually explained by rounding:
- Total carbohydrates: 11.45 (rounded to 11)
- Sugars: 8.55 g (rounded to 9 g)
- Fiber: 2.55 g (rounded to 3 g)
...which would mean there are additional 0.35 grams of non-sugar, non-fiber carbohydrates...
...using such a label with only rounded values (and students not knowing the exact values), does not make it possible for students to calculate the amount of non-sugar, non-fiber carbohydrates.
So, another explanation is that there is an error in either total carbohydrates (11 g - too low) or in sugars (9 g - too high; more likely 6 g), according to NutritionData for 150 grams of raw onion:
- Total carbohydrates: 14 g
- Sugars: 6.4 g
- Fiber: 2.6 g
In either case, it seems they have used a nutrition label that is inappropriate to ask a question about the amount of non-sugar, non-fiber carbohydrates in raw onion.