Variance
In probability theory, the dispersion of a normal distribution about its mean is called the variance. The variance of a random variable is a measure of how far the value of that variable will deviate in the short term from the long term expected value, or mean.
In poker, which contains an element of randomness, the variance refers to a player's deviation from his expected winnings. For example, a flush draw on the flop should win 36% of the time, but averaged over a small number of hands it might turn out to win only 25% of the time, or the flush might be missed five times in a row.
The difference between a good and bad player will not be that the good player wins every round; sometimes the bad player will get lucky. He will appear to win because small deviations from the ultimate outcome fall within the variance of the game.