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People who are ideologically conservative are more likely to vote than those who are liberal. The survey is based on the average historical midterm election turnout. This means that the numbers may look different for a presidential election.
I would say this has to do with level of awareness...
I would be inclined to agree. Additionally, many liberals tend to be younger and generally less politically engaged. They may associate themselves with a liberal philosophy, but they don't necessarily have the political fervor to turn out in large numbers come election time.