It is impossible to measure the blood glucose with meters without variability. Two tests within seconds from the same finger will differ in their result. This is caused by the measurement principle: a catalytic process is triggered by the blood glucose in the blood drop. The result of this process can be measured electrically. Reasons for little differences in this reaction:
a) random variations in the production process of the stripe
b) different fluid concentrations depending on the deepness of the prick
c) relation of haemoglobin to fluid in your blood: most manufacturers make an assumption about what concentration is normal. It is obvious that this relation changes for women in their period.
d) other random influences like air pressure or temperature
Because of these reasons ALL of todays meters will have their variations (even the lab machines). The good news is that all meters in the market must have a specified reliability. You will find this information in the description of your test stripes.