InventoryClickEvent doesn´t fire?

Hey
while writing a Shop Plugin, i think i encountered an error.
The InventoryClickEvent doesn´t fire on every Inventory Click.

http://i.imgur.com/XwnDZo9.mp4

Here´s a gif, where you can see a message in the chat upon every click. But as i opened the chest, i made it to place two items (the snow and the grass block) in the chest where the Plugin just detected 2 clicks (but there were 4).
Here´s the code for the event:

Code (Text):

    @EventHandler
    public void invClick(InventoryClickEvent e){
        e.getWhoClicked().sendMessage("Inv :"+e.getClickedInventory().getName()+", slot: "+e.getSlot());
    }

Is it just me (am i overseeing something) or why doesn´t the event fire on every click?

 

I hoped you were right, but sadly the same thing happens in every game mode. :/

Yes. If i wouldn´t have registered it, i would never get a message.