Item distribution in several dispensers/droppers using hoppers depends of non dropper/dispenser touching hoppers.

-If 2 dipensers/droppers are one above each other and if hoppers sending items into them form a strictly vertical row, then item got shared in the same quatities in each dropper/dispenser.

-If 2 dispensers/droppers are one above each other and if hoppers row sending items into them contain an horizontal part, then item will all goes into the highest dispenser.

-If many dispensers are one above each other, and hoppers in a strictly vertical line,item will got shared in the same quantity, but only in the highest and lowest dispensers/droppers (with the adjacent green wool on the screenshot).