In "The Lottery," it is highly improbable that Old Man Warner would survive into old age. To illustrate this point, consider that the lottery takes place every year so Old Man Warner has survived seventy-seven lotteries. This is an impressive feat, made all the more improbable by the village's relatively small population. With only three hundred residents, Old Man Warner has continually overcome the high odds of being chosen.
With this in mind, the reader might expect that Old Man Warner's time has finally come. After all, nobody else in the village has survived anywhere near as many lotteries as he. In a twist of events, however, Old Man Warner has another lucky escape when Tessie Hutchinson is announced as the winner. Jackson, therefore, suspends the normal laws of probability to create a twist ending.
A fine question. Given the size of the town (as indicated in the story), I'd have to say that it is the fact that Old Man Warner hasn't been chosen.
At one point, we're told the following:
"Seventy-seventh year I been in the lottery," Old Man Warner said as he went through the crowd. "Seventy-seventh time."
Given the limited number of names called, he should have been stoned to death a long time ago.
Now, emotionally, the chance that seems least likely is Mrs. Hutchinson first forgetting, then getting chosen.