The reason is that from Twin Diamonds, the ends (points) are considered to be Ends of Lines, and when a shape-changing call is directed to the centers, "square breathing" causes the Ends to laterally slide toward or away from the center.

From Twin Diamonds, if the Centers do a Lockit, the Ends slide away from the center, but remain close to each other (in Tandem) as if they were Ends of Lines. This results in the "dogbone" formation.

Ceder Chest Definition of Lockit