Half life is a mathematical term, a statical term. Because the random nature of decaying isotopes we use statistical models to gain understanding of the phenomenon. The half life of a radioactive material is approximate amount of time it takes for a sample of material to be transmuted. And like all statistic models (at least good ones) the larger the sample size, the closer to the actrual number.

Think of it this way if you had two tons of plutonium 239, and you had all the necessary equipment to "count" how much plutonium you had after 2.44 x 10^4 years you would have one ton left (+ or - a couple of grams, maybe a kilogram). Now do the same experiment but use only two atoms, there are no guarantees that after one single day or even a single second that there would be two plutonium atoms. On the flip side 60,000 are even 80,000 years could pass and none of the plutonium atoms have decayed. It's a random process that we apply a mathematical model too, for large amounts it works great, the smaller and smaller the amount the less accurate it becomes, remember that.