Cool! That explains why the cup isn't full after transferring the coffee, even though the size of the pot is likely the same as the cup.
It's similar to how a half-full bottle of carbonated drink appears full after being shaken, due to the foam created by the released gases.

Distributed heat (sand) => coffee releases gases => water becomes foamy => foam is transferred to the cup first => remaining coffee is poured into the cup: pretty much it