Zero sum means there is a fixed amount that is distributed between all participants.

Inequality means that the distribution over available resources is uneven. Inequality can occur in the presence of growth if the gains from growth are not distributed equitably. I.e. inequality does not imply zero sum.

To the extent that economics agree on anything they seem to agree that wage growth has stagnated since, roughly, the 1970s, inequality is rising, and gains from growth are disproportionately going to the richest.

Finally, the census measurements capture data at an instant in time. At any one instant there is a fixed amount of $s in the economy and it's a mathematical fact that comes from the definition of median that you cannot have samples (i.e. income) higher than the median without having samples lower than the median.