[GrekeHaus]

If you're looking for a more intuitive reason that SNGs have less variance, you can look at it this way:

Whenever you are playing a SNG, you profit every time one of your opponents makes a mistake. In some sense, you profit from every hand that you're not involved in. Conversely, your opponents all benefit from every mistake you make. So if you're making fewer mistakes than your opponents, you will hence make a profit.

The net result of this is that you get more meaningful hands per hour when playing a SNG. Compare this to ring games, where you're folding the vast majority of your hands and so the ammount you win/lose might only be determined by 2-3 hands per hour. Whereas for SNGs, there will be many more meaningful hands per hour because a lot of the meaningful hands will be ones that you're not even involved in.