Originally posted by Psojed:

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.

The game has allways shown the total amount of a single recently picked up resource. Picking up 2 Woodstacks of 50 displays that 100 Wood were added to the inventory. It's not the stacksize, that is faulty, but the pickupdisplay.

pipo.p has it right, I didn't look on the known issues page before posting.