Systolic blood pressure[SBP] depends on the cardiac output. When Nor adrenaline is given there is vasoconstriction due to alpha-1 action on blood vessel, vasoconstriction results in increased total peripheral resistance and thus increases the diastolic blood pressure. The beta-1 action [increase in cardiac output] on heart by Nor adrenaline is counteracted by the reflex bradycardia resulting in decreased cardiac output. Given the fact that SBP depends on cardiac output and the cardiac output is decreasing, why is there a rise in SBP on administering nor adrenaline?
Your question involves comparing an observed response to administration of noradrenaline/norepinephrine, which causes vasoconstriction thereby increasing the BP (during both systole & diastole,) to the hemodynamic/pharmacodynamic response of the cardiovascular system as a whole, which is decreased cardiac output & decreased cardiac work. This is similar to asking why opening a finite water tank eventually slows down even though the pressure increases when you turn on the faucet.