Technically, they call it a "weeks estimator" to alleviate concern with exact accuracy. Even on their website, they claim it is only an estimate and only a true blood test could give the most accurate information. Mine has been on 2-3 weeks the whole time so far.
Here it is directly from their website
The Weeks Estimator result is determined by the level of hCG in your urine. The level of hCG varies from woman to woman and therefore the Weeks Estimator may give misleading results. All results should be confirmed by your doctor, especially when making decisions about future medical care. Only your doctor can determine whether your pregnancy is healthy.
Your doctor determines how many weeks pregnant you are based on the first day of your last menstrual period and ultrasound results. This test provides a different estimate. Please note that the doctor may date your pregnancy differently from the information shown in this table, since pregnancy dating is dependent on the circumstances of the patient.
Agreement of Weeks Estimator results with clinical findings ranged widely from 45-99%.