I know in a few rounds we could probably come up with an explanation that is clear and simple.
We have these 10 Zones in our subject that the meter says are 10 stops apart from black to white in our scene (give or take).
The black in our scene is blacker than the blackest black on any print. Because the black in our scene is in the shade.
We use the Zone System to make these 10 Zones fit our print so it goes from blackest black paper to whitest white paper on the print.
But you're not supposed to go and use the light meter to measure the print... I want to make it clear that it is not necessary, and that it is absurd.
You're just supposed to see that the print goes black to white (as much as you envisioned).
Anything 2-dimensional in even lighting, will only measure about 7 stops difference. The reason our scene measures 10 stops or more sometimes is that a lot of scenes have parts that are in the shade.