A mathematical irony

[mathematic]

2. I think you're doing the math wrong

It seems like you're assuming these odds are independent and thus you can find the probability of winning any particular set of states by multiplying the probabilities of winning the individual states together.

If you look at Silver's "Scenario Analysis" bar chart, you'll see that the chance for going 51 for 51 (well, 332 electoral votes to be precise) was just over 20%. By eyeballing it, you'll see that 50/51 adds up for at least 30% more.

To explain a little more with some intuition, let's say I assign a probability to Obama winning OH. I later find out that Obama has won VA. Based on this information, I'd increase the probability I have of Obama winning OH. I do this because a lot of the unknown, random, or otherwise unaccounted for possibilities that would favor a Romney win in OH are reduced or eliminated by knowing that Obama won VA.

In particular, Silver's methodology specifically includes national biases that break one way or the other.