Whenever you pick a big enough stack from the ground while also having a partial stack aleady in your inventory, the picked-up stack gets split into two inventory slots, and the game displays it as picking two different stacks, hence why you see a different number.

ie.
pick 36 -> inventory has one stack of 36 wood -> game displays 36 in the top right
pick another 50 -> game adds 14 to the first item stack in your inventory to reach maximum stack of 50, and creates a new inventory stack with the remaining amount 50-14=36 -> game displays 36 in the top right

It will never display 72 wood, because the maximum stack of wood is 50.