First, the absolute accuracy of the zener voltage is poor. So it has to be measured with an accurate voltmeter before use.
Even the mighty LTZ1000 used in the best multimeter in the world has very poor accuracy, however it's extremely stable.
It doesn't matter if the output is 7.034523V or 7.245321V, what really matter is that if the output at one given moment is 7.245321 it STAYS precisely at that value no matter what.
Once we know the voltage is stable, we can compare it with calibrated voltage standards to get the exact number.
Every multimeter and voltage reference works this way, you can't just built it and expect to be accurate, at most you can expect to be precise (that is it does't change its value)