Even if it is not disproportionately so, this is still not surprising. If you look at the early stage of Presidential election years, opposition front runners invariably start off with a poll advantage over incumbent Presidents. Mondale lead over Reagan, and Dole did over Clinton. It is the unknown factor. I would be really worried for Kerry if he did NOT lead Bush at this early stage.
Once the candidate is defined by the incumbent, it is a different story.