Guild Wars 2

Could someone please explain why:
If I have a stackable item in my inventory (lets say 1 Giant Wintersday gift) and I want to drag and drop it on a partial stack of Giant Wintersday gifts already in my Personal bank(currently 12 gifts), the gift is added to the partial stack at 1 + 12, no problem.

Now if I have the same scenario but this time I try to drop the 1 gift from my inventory into the Guild Bank of 12, it gives me a red box and does not allow it.

So I have to pull the 12 from the guild bank into an empty slot my personal inventory then drag and drop either the 1 or the 12 to the other stack and THEN move it back to the Guild Bank (it won’t let me drag and drop on the 1 gift I already have in the Personal Bank and it wont let me combine the stack within the Guild Bank).

Now my question is what am I missing?
Does this bother anyone else?
Is there a setting or something to change this?
Why is it ok in one bank but not ok in another?

Signed,
Confused in Tyria

I believe this was implemented to prevent item duping where multiple people have access to the same stack of items simultaneously.

One bank permits access by one player. The other bank permits access by multiple players, up to 500. Many actions could happen at once in the bank which allows multiple access.