The stocks moves behavior in Odoo is similar to double-entry accounting. 

It means that an "IN" move in a location needs a "OUT" move in other location. 
When somebody makes a stock adjustment, for example increasing the stock, an opposite move is created.  In  this  case,  a  negative  move  in  a virtual  location   ("Inventory  adjustment"). The same occurs when somebody makes a reception from a vendor. 

Usually, the virtual locations are hidden in the views. You have to change the filters if you want to see them.