Huh... It's like the cursor remembers how many objects are in the stack most recently drag and dropped until a container is opened. I started with a stack of 30 sling stones that I dropped into a container. Then I picked up 3 individual stones and dropped them, one by one, into the same sack (it doesn't seem to matter if it's the same container or different ones). When I opened the sack, there were four stacks of 30 stones.