The article's final paragraph contains the explanation for this data, but doesn't explicitly connect the dots: Nielsen numbers fail to include other forms of viewing besides families watching at home, particularly debate parties, Internet streaming, and deferred viewing on Tivo, DVRs, or the Web.
Nielsen is an institution based in an earlier media formation, when liveness and immediacy were the hallmarks of TV, were the features that distinguished it especially from film or print. In some of its earliest manifestations at the end of the 90s, the Internet remediated TV's liveness through web-cams, live video streams, and so forth. And today, via Twitter, blogging, and other forms of live chat, the Internet continues to assume some of TV's mantle of immediacy. A related media shift has been marked on by many observers in relation to polling data, which like the Neilsen ratings, is grounded on media and telecommunications patterns that do not capture the dynamic usage of a significant portion of Obama supporters. The liveness of the home telephone has been increasingly remediated by cellphones and texting.
In today's era of premediation, however, networked media users are less committed to liveness and immediacy. So much of our TV universe persists well beyond its initial broadcast that TV "viewers" are perfectly content to watch all or part of important shows on YouTube or other video-sharing sites. Viewers can be assured, for example, that if they miss Tina Fey's spot-on remediation of Sarah Palin's interview with Katie Couric, they will be able to find it on the Web. Obviously the same is true of the presidential debates--or of the coverage of the political conventions, where observers were similarly surprised to note that the Republican convention was viewed by more people than the Democratic convention (which may have been the case, but the number of Obama supporters who viewed his speech in another medium is most likely significantly higher than those who similarly viewed McCain's or Palin's speeches).
Because live events are already premediated in any number of different media formats (even before a live event is televised its availability on YouTube and other networked media is always assured), institutions like Neilsen and other mainstream media will continue to have to find new metrics to measure today's and tomorrow's ever-changing media audience.