By law, a blood glucose meter can vary by 20% - that means if your BG is 100 your meter can read anywhere from 80 to 120 and the meter is considered to be just fine by the FDA. At 138 the meter can read between 110 and 166 and still be considered accurate. Test strips are very sensitive to heat, light, moisture and age- so any of those factors can make your strips perform differently. Some strips are calibrated to whole plasma readings and others are not. The One Touch family of meters is calibrated to whole blood to me more in line with the results from meters used in clinical laboratories.
One of our senior members posted awhile back that there is a study that showed that different fingers even give different numbers - even if the readings are taken at the exact same time. Glucose meters are definitely NOT precision instruments and your BG readings should be used to look for trends rather than absolutes. Otherwise, you'll drive yourself crazy-
Sandy