Three kanban cards are hanging on a board next to a work area, while another identical kanban card is attached to the container that an employee is busy filling with parts created in that work area. Five more matching kanban cards are already attached to full containers placed in a convenient waiting area near this work area. A supervisor walks over, removes two of the kanban cards from the board, and puts them in her pocket. The supervisor then walks away. Which is the best explanation for why the supervisor did that?