The number of carbs in something can vary. For cakes, for example, you may get a slice with more currants than the next person.
For "natural food" there are many things which can affect the carbs: the soil it was grown in, how ripe it is, the species, ...
And some books/websites include fibre and some don't (usually in the Uk, we do not include fibre but we may inadvertently go to a US website).
The only way of telling exactly how much carbs there are in food is to burn it and then weigh the resulting carbon.
But this doesn't help if you want to eat it.
Therefore, all carb counting is an approximation. And, considering there are other things like exercise and stress which affects our BG, these approximations are not too bad.
I think most people do not eat anything for the time of their basal tests.