USBLC6-2 Zener Breakdown Voltage
Well, actually those Diode that you are looking at are not zener diode but rather TVS diode, TVS diodes do not have as sharp of a curve as a zener diode does are intended to take fast power surges, while a zener diode is intended to create a sharp curve at the zener voltage. If you see the data sheet, it states two voltages that are Vbr = Breakdown voltage and Vcl= clamping voltage. The breakdown voltage is the voltage at which the TVS will begin to do minimum conduction, specify in the datasheet as 1mA. And the Vcl is specified at 12V with a 1A and and 17V at 5(remember that its ESD protection and during and ESD event 1A and 5A for a few pico seconds is not out of the question). This Vcl is the actual voltage at which the TVS diode will clamp.