In addition to the margin of error on food labels, there's also the margin of error on blood tests, which IIRC from the spec's on the one my wife uses it too is about +/- 20%.
If you have the misfortune of having those errors match up (and they will at times) they you eat something that overestimates carbs by 20% and a blood test that overestimates your current glucose level by 20%. Give yourself a corresponding dose of insulin and drive your levels through the floor.
If you have the misfortune of having those errors match up (and they will at times) they you eat something that overestimates carbs by 20% and a blood test that overestimates your current glucose level by 20%. Give yourself a corresponding dose of insulin and drive your levels through the floor.