The reason is that we are now getting to the point where a neutral day in the polls can be thought as being ever-so-slightly favorable to Mr. Obama, since he leads in the race and since Mr. Romney now has only 45 days to make up the deficit. This will be especially true over the course of the next week or so, during which time the penalty that the model has been applying to Mr. Obamas polls because of the potential aftereffects of the Democratic convention will phase out.
Another way to look at this: Mr. Obamas win probability in our now-cast, which is our estimate of what would happen if the election were held today, is 95.8 percent. (The now-cast also does not apply the convention bounce penalty to Mr. Obamas polls.) As Election Day draws nearer, the forecast will converge toward the now-cast, until eventually they are exactly the same on Nov. 6.