Updated January 01, 2014.

Why do carb counts vary so much? Readers ask me this question as they look at sources on packaging, on the Web, and in books. There are a few reasons why these figures can be different from each other.

Food Variability

We'd like to believe that every single large strawberry has exactly one gram of carbohydrate, but common sense tells us this isn't the case. The values in the various databases are averages of many tests, some of which vary a lot.

Growing conditions, species, and even types of fertilizer all have an effect. Even packaged foods are allowed to vary 20% from the values on the label according to FDA standards (and tests reveal that they sometimes vary even more).

Different Databases

In the United States, most nutritional information is taken from the USDA National Nutrient Database for Standard Reference (usually just called the USDA Food Database or USDA Nutrient Database). But some other countries compile their own databases. The nutritional software I use combines the U.S. and Canadian databases (handy when a food is in one database but not the other).
Additionally, every so often the USDA comes out with a new "release" of the database, with updated information. As more information accumulates, the values change, usually not by a lot, but sometimes by quite a bit. They also make adjustments for changes in foods sold in stores. For example, the current release of the database has new descriptions for small, medium, and large apples, all of which are larger than the ones of even a few years ago.

Changes Over Time

The food we are eating now is not the same as it was even a few years ago. More sugar and salt has been added to processed foods such as cereals and breads over the years, with some of these foods containing as much as double the sugar of the same products 30 years ago. Even "natural" foods are different. For example, many fruits have been bred to be sweeter and more appealing as time goes on. Cantaloupe almost doubled in sugar content between 1950 to 1999.
As you can tell, it's no wonder that we see different carb counts in different tables and on different packages. So what are we to do? Perhaps ideally we would check our own blood sugar to find out our reactions to foods. Besides that, I think that using the USDA database and/or packaging materials is fine. Just keep in mind that we can never know exactly. A good source online source of information is Calorie Count Plus, where you can plug in a food to get all kinds of nutritional information about it.

<sub>Sources:

Davis, Donald, Epp, Melvin and Riordan, Hugh. "Changes in USDA Food Composition Data for 43 Garden Crops, 1950 to 1999." Journal of the American College of Nutrition. 23/6, 669-682 (2004).

Leroux, Marcus. "Give us this day our daily bread . . . but why put so much sugar in it?." Times. London, England. 07 05 2007.

USDA National Nutrient Database for Standard Reference, Release 20.

USDA National Nutrient Database for Standard Reference, Release 18.</sub>