Every time, any inventory is opened (self or a storage of any kind), the top left item is selected by default regardless of where your cursor/pointer is creating confusion.
So if you were to open your inventory and immediately press Shift + Left Click, your helmet will be automatically removed from your head even if your cursor was pointing at an empty slot.

Steps to reproduce -
1. Open your inventory and don't move your pointer
2. You will see that your pointer may be pointing at a different item but he info-box will display the details for the item in the inventory's top left corner