notice how this time they qualify it. A color is given if the candidate wins by 5%. I guess they wanted a 3rd color in there so it wouldn't look so clear.
My interpretation is that it marks the counties that were 10000 votes for one candidate, 10001 votes for the other.
As a result, it shows that there are an awful lot of counties out there that have at least 5% more Bush votes than Kerry votes.