The overflow works like this. Let us say you have characters on two worlds with 60 items in the shared storage and only 60 open slots. When you transfer them to one world, you may have 120 items in shared storage with 60 open slots, since the 60 slots is what you've opened on all available worlds. Opening the shared storage will show 120/60. You will be able to remove items from the shared storage but not put any into it until the total has been reduced to 59 or you buy unlocks for more open slots. Let us say you buy one more shared storage upgrade. You will now see 120/75 if you buy the unlock before moving things out.
Oh, if I'm not mistaken, stackable items will merge up to their maximum stack size; so items like that may result in fewer slots shown as overflow.
"No sadder words of tongue or pen are the words: 'Might have been'." -- John Greenleaf Whittier
"Do or do not. There is no try." -- Yoda
On planet Earth, there is a try.
Indeed, in a world and life full of change, the only constant is human nature (A is A, after all :P).
We old vets need to keep in mind those who come after us.