The Systolic and Diastolic blood pressure are measured relative to the atmospheric pressure, hence they are "stronger" than atmospheric pressure.
The blood pressure measured (Systolic and Diastolic) respectively measure the maximum and minimum pressure between heart beats relative to the atmospheric pressure.
As John Custer noted in the comments, this type of measurement (relative to an external pressure) is referred to as gauge pressure. An important distinction to make is that gauge pressure measures the pressure of the system with respect to the pressure outside of the system; therefore, the external pressure is not always atmospheric pressure. Additionally, in some cases it may also be referred to as 'relative pressure' though this would be in a more colloquial setting.
Blood pressure, (and gauge pressure) is measured this way to formulate a numeric representation of the blood pressure independent of external factors. As an example, atmospheric pressure fluctuates with weather and height. If it were measured absolutely, then the 'average' blood pressure would vary over the year and with height. By measuring the gauge pressure, such distinctions are limited considerably, but not removed entirely. For example, your blood pressure would still increase when you go up a mountain; but, now when your blood pressure is measured, the difference is made much more apparent.
- Magder, S. A., MD. (2014). He Highs and Lows of Blood Pressure, Toward Meaningful Clinical Targets in Patients With Shock. Crit Care Med., 2014(42), 5th ser., 1241-1251. Retrieved June 08, 2017, from http://www.medscape.com/viewarticle/823925_2