There may be two reasons for the same.
(2) Number nine is considered as intact.
From amongst all the numbers from one to nine, the only number which is intact/integral/ceaseless/integrate is nine.
When any number is multiplied by nine, then the number which we get at the end as an answer is nine
 As for example : 9 x 1 = 9
9 x 2 = 18 ; 1 + 8 = 9
Similarly if three, four, five, etc. are multiplied by nine, then the respective answers for the same would be 27, 36, 45 etc. So, if any number is multiplied with such a number whose digits when added would sum-up to give you an answer of nine, then such a type of multiplication would always lead to an answer whose digits when added would also sum up to nine.
E.g. 27 × 26 = 702 = 9
 Let’s see one another artifice.
Guess any one number. As for E.g. 332. Now sum up the three digits of this number. So, the answer will be eight. Now subtract this number from the original number. So, if we subtract eight from 332, then the answer that we get is 324. Now add these three digits. So, by adding the three digits of three hundred and twenty four, the answer that we get is nine.
Thus if such a type of game would be played with any number, then the final answer that we would receive, would eventually be nine.
 Let’s see one another artifice.
Think of a number. Now, write the number in the reverse order. Now when you will perform subtraction between the original number and the reverse number, you will get nine as the answer.
E.g. Twelve is the original number. Now, if you reverse it, then the number that you get is twenty one. So, by performing subtraction on these two numbers, the answer that we get is nine.
 One last artifice.
From amongst the numbers of 0 to 9 : Add up the smallest and the largest number which would be coming up consecutively from both the ends. You will get nine as the answer.
E.g. 0 + 9 = 9, 1 + 8 = 9, 2 + 7 = 9, 3 + 6 = 9, 4 + 5 = 9.
 In addition to these nine has been used in many places.
Whenever Tirthankar walks, at that time nine gold lotuses are placed them on the pathway below His feet. There are in total nine living as well as non-living elements/essences (Nav Tatva). The total count of Panch Parmesthi’s virtues comes up to 108 and the total of this number is nine.
The baby lives in the mother’s womb for nine months. There are nine main planets orbiting the sky.
Out of the 24 Tirthankar’s, the names of the twelve tirthankaras are made up of four syllables and the names of the twelve tirthankaras consist of five syllables. So, the total summation of number of letters that come up in the names of the twenty-four tirthankaras is 108. If we add up the digits of 108, then it comes up to nine.