When you have anything with space (ammo box, clothes, etc.) in your inventory and you pick up something from the ground, it goes inside the item with empty slots.

Like this, you can place, for example, food in ammo box, and anything in to a 6 slot jacket that is in your inventory, while it only takes 4 slots.

I had an extra pair of tactical shirts in my inventory (takes 1 slot, has 4 slots) and noticed that i can't move them around my backpack. I had to drop the item to the ground, and noticed that it was full of items I had previously picked.

*EDIT*

Apparently only happens when you do actions with an item, such as 'open', 'craft', 'unpack' etc.