If I'm not mistaken this is a DirectX 11 feature. The game has baked reflection maps for reflective surfaces, which can create a very detailed effect for static objects, but is far more limited for moving objects. As you observed yourself, the reflection on the weapon is actually a generic reflection map based on the environment you're in, and doesn't actually reflect your immediate surroundings. At a glance it adds a lot of realism but on closer inspection you can see through the trick. Any mirrors added to the game for housing would have to function in the same way as the reflections on that gun as placeable housing objects are non-static and thus can't have baked reflections.
