The setting DisplayGeometry sets the resolution used by the X server when it starts, and it is kept unless the client asks a different resolution.
Normally settings configured on server side take over those configured on the client, but in some cases the client has priority. The geometry is very specific for each client configuration, and forcing it on server side may cause serious headaches for users with a different display resolution. This is not your case, but consider a server configuring a resolution 1920 × 1080 and a user connecting with a laptop 1366 x 768. He would be forced to use either the fit to window to scale the session, or the viewport to navigate the session with scrollbars.