It all has to do how they are shown on the SD broadcast (remember, they comprise the largest audience). For Leno, NBC centercuts the 16:9 and crops out the center to 4:3; hence, everything is framed for 4:3 and anything on the sides is "extra space". For Conan, his show is letterboxed on the SD broadcast, so they frame for 16:9. Some widescreen movie clips are upconverted and shown full-width, though there have been times when Conan gets HD clips. For Leno, even the letterboxed clips are pillarboxed to make it easier for a 4:3 SD centercut; to us HD viewers, we see black bars all around!