I did read it here where Last Light used supersampling AA instead of the normal one
http://www.geforce.com/whats-new/guides/metro-last-light-graphics-breakdown-and-performance-guide
With a bit of technical know-how and the right hardware you can already downsample in games from resolutions higher than that of your screen, such as the aforementioned “4K” resolution,
3840x2160, increasing visual detail considerably.
Last Light's SSAA has a clear advantage, however, in that is built directly into the game code, is optimized specifically for Last Light, has a much lower performance impact than other downsampling techniques, and can correctly anti-alias edges unlike general downsampling achieved through GPU scaling. In those cases, additional in-game or injected post-process anti-aliasing is required, further lowering the frame rate.
Metro: LL's supersampling had less of a performance impact than the MSAA used in Metro 2033.
The difference here is that it was built into LL's game code so I had misunderstood.