Probability theory describes the possible deviations from the expected normal distribution of a random value as its variance. The variance of a random value is, so to speak, a measure of how far a value can deviate in the short term from the long-term expected result or mean.
Translated to poker, which contains a random element, variance describes the possible, short-term deviation from the expected long-term expected value of a player. For example, in Texas Hold’em, a flush draw on the flop will give it a flush on the river in about 36% of the time – long term. With only a few hands in the game, he may only hit the flush by 25% and miss his draw five times in a row.
The result of a consistently good player against a consistently bad player will rarely be that the good player consistently earns profit in every round of poker. Sometimes the bad player is just lucky and the variance is on his side.