problem for those good at math

In his will, a nobleman leaves half of his horses to his oldest son, a third to his second son, and a ninth to his youngest son. At his death the nobleman has 17 horses, so none of these bequests results in a whole number of horses for any of the sons. The executor of the will solves this dilemma by adding one of his own heroes to the estate, making a total of 18. Now, according to the provisions of the will, he gives 9 horses to the oldest son, 6 horses to the second son, and 2 to the third son. Thus, each son inherits a little more than he was originally entitled to, and one horse is left over, which the executor takes back again. Everybody is happy, but something seems frond, what is wrong?

Can you explain clearly, if possible