Poker is a high variance game by its very nature. Due to the sheer number of possible combination of cards coupled with the fact each outcome is based on odds and probability, luck plays a major part in the result in the short term, though skill prevails in the long run.
Same forms of poker are higher variance than others. For example, cash games with a fixed-limit betting structure are higher in variance than no-limit variants due to the fact you usually see more showdowns and have to have the best hand to win one. But in no-limit games you, even though you can win or lose your entire stack in one hand, you can win pots easily without a showdown which reduces variance.
Tournament poker, particularly multi table tournaments, have almost unbelievable variance attached to them. This is mainly down to the fact you are up against not just a full table of ten players but often hundreds or thousands of other players, which reduces your chances of winning dramatically. There are also more showdowns and all-in encounters in a tournament scenario and even if you were only all in three times and each time you had Aces against Kings, making you an 82% favourite, you will only win all three encounters 55% of the time! Just imagine how many times you are all in during a typical tournament and you will easily see why it is so difficult to win one!
One way to reduce the variance is to increase the number of tables you play. Doing this allows you to play more hands, which in turn means odds and probability figures will converge towards what is mathematically expected much quicker than if you just played a single table.
Most online professionals who play poker tournaments for a living play 10-20 tournaments at once so that they can try to reduce their variance. In a typical week they may play a couple of hundred tournaments with this method where your recreational player playing a single table may only manage 10. It is not uncommon for tournament players to go 30 games without reaching the money which would be two days work for someone playing multiple tables but three weeks for our single tabling hero.