There is a certain amount of good will that emerges as the structural problems with politics become clear. When a president is in office, they tend to receive all the blame, or credit, for what happens in Washington. When there are problems moving forward a policy, such as in reviving the economy, the public tends to blame the president. But when those problems continue long after a president is gone, as has been the case with the economy, the public tends to gain some perspective (even as they attack the existing president) about how the sources of discontent stem from the structural problems with the government and the economy rather than any individual person.
We judge a president based on the political times we are living in — not simply in the times that they governed. Presidential reputations could improve if the polls are tallied at a moment when a president resonates with the politics of the moment.
Finally, and this is not relevant to Bush yet, the opening of archival records from the period a president is in office can dramatically change how we see him.
President Dwight Eisenhower, who was once treated as a popular yet bumbling president, was found to have had a firm grip on decision-making in the White House. Discoveries from the Ronald Reagan archives in Simi Valley, California, revealed that he was much more than a former Hollywood actor who moved to the top through charisma and luck.