That is the whole insurance business. It is nothing but a bet if you look at it by a single policy. E.g. your insurance bets you'll probably die at 70, so when you hit 71 you paid more than you get when you die. Your insurance wins. But when you die at 55 you "win" the bet. But because there isn't just one but thousands of policies those bets aren't bets anymore, they're nothing but statistics. And one part is keeping track and calculating how old people born in year xy, in country ... with the gender zxy are going to be.