That's why. You need to specify that a given Software Update Plug-In policy (agent config) can override the Maintenance Window.
To check your Maintenance Window settings go to Settings > Agents/Plugins > Symantec Management Agent > Settings > Maintenance Windows
See the attached screenshot. You'll need to check the checkbox at the bottom of that image in order for the Software Update Agent to override the Maintenance Window or else it will patch when the Window opens like regular behavior.
Maintenance Windows allow you to queue up actions against a system to allow them to happen only during the defined window, regardless of when those actions are scheduled.