Possible but not very likely explanation: when the polls say there is say a 4% margin of error, that means 95% of the time the error will be within 4%. But there's still a slim chance, 5% in this example, the error will be larger.
To illustrate, let's say there are 500 people being polled out of 5 million voters. Then there is a tiny but non-zero chance that you would, for instance, only get Romney voters, and he would poll 100% and everyone else zero.
To illustrate, let's say there are 500 people being polled out of 5 million voters. Then there is a tiny but non-zero chance that you would, for instance, only get Romney voters, and he would poll 100% and everyone else zero.