Well, actually, they just didn't add an important aspect. This is how it all really went down:
There was an old farmer with three sons. When he died, he wanted to spread his 17 cows amongst his sons. He wrote down a will stating the oldest son will get half of the cows, the second son will get one third, and the youngest one will get one ninth. After his death, the three brothers sat on the farm for days not knowing what to do. One day, their neighbour came along to see how they were doing after their father's death. When he heard of their issue, a smile rose on his face and he ran away to his own farm, saying he'll be back soon. He soon returned with one of his own cows with him. 'Now,' he said, 'as we have 18 cows, the math gets easier. The oldest son gets half of the 18 cows, which is nine. The second son gets one third of the 18 cows, which is six. And the youngest son gets one ninth, which would be two, accumulating a total of 17 cows. And one cow, my cow, will be left over.' He then left with his cow, wishing the boys the best of luck.