The glass bell display a incorrect number of items in the bell.
If you open the GUI and put in 2 items then remove one by right clicking, the glass bell will display the item like their were multiple ones. The same happen if you place one item in the glass bell GUI and then one more, the glass bell will now only display like their was one item.
It seem that the model is not updated when the stack size changes, only when the itemstack is replaced.