Zener Diode Voltage and Curve Equation
The data sheet that you linked shows that the "Test Current" for the 12 volt zener is 21 mA. 

You tested at a "Test Current" of 67.5 mA. You should re-test at 21 mA (zener current).  

EDIT :  A comment asked if the "breakdown voltage is not constant".
     The graph below shows that the breakdown voltage is not constant, but varies with the test current (near where VZ is marked in red).