To the OP, both the Cd's and County numbers have to do with the fact that the Democrats are FAR more concentrated than the Republicans are. Obama winning Los Angeles county with about 70% of the vote gave him a bigger individual county margin than something like the top 20 or so McCain counties combined.
On top of that, Democrats have black voters, who are vastly more concentrated in their localized voting habits than any other demographic in the United States. The GOP has no parallel to say, Central Brooklyn, where the Democrats could draw a full congressional district that voted like 98% for Obama. The best i've seen is an ugly mess in West Texas around 80% McCain, and that's only in one part of the country. Democrats get over 90% margins in a ton of places around the US (northern Miami, Southern Dekalb, Chicago, Detroit, etc.) and this generally concentrates their voters in a way that gives the Republicans an edge in a 50/50 election, even with fair maps. Every 80-20 District (which the democrats have like 30 or so) represents a 60% margin wasted because of concentration.
And that concentration of Democrats gives the Republicans a structural advantage in terms of the Electoral College and Congressional districts.
