According to the press release on this poll, these are the numbers you get when you force the poll respondants to choose one of the "big six" (Clinton, Obama, Edwards, Giuliani, McCain, and Romney). They also have separate numbers for an open ended question in which the pollster asks the respondant who they support without prompting them with any names.
Either way, it's not directly comparable to other polls, in which the pollster will give the respondant the list of all the candidates running, not just the top three from each party.
