Why do SNG's have less variance?

Quoted from the FAQ:
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Why play SNGs?

Different people have different reasons. Some people like the “it’s a science”-ishness of them. Some people like the fact that they take a pretty standard amount of time. Some people like the variety of going from full table all the way to heads up. Some people just got burned out on whatever their last game was and think SNGs are fun. Others like the fact that the variance is lower in SNGs than other forms of poker. It’s all good.

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Why is it the case that SNG's have less variance than ring games? I find this to be true too, and I'm not sure how to explain the reasoning for this. What are your experiences/explanations as to why this may be true?