Why in the multiplication table, different characters such as 如，得，歸，中 are used? It seems to me that the following rules apply:


When the answer is a one-digit number, the character 如 is used.
When the answer is 10, the character 得 is used.
When the answer is a multiple of 10, other than 10 itself, the word 中 is used.
When it is 3×3=9, the character 歸 is sometimes used (though not on this picture here).
In all other cases, no additional character is inserted.


I am wondering why there are such rules? Do we use these rules anywhere else?
九因歌 using synonyms words instead of using one character universally is to make it 押韻 rhyme better, easier for children to 'sing' to remember it better
如 = as same as
得 = get
中 = together
~
二五得一十 ( 2 X 5 get 10 )
二三如六 (2 X 3  is as same as 6)
四五中二十 (4 X 5 together is 20)

歸 = end up with
三三歸九 (3 X 3 end up with 9) is a variant of 三三如九. No other set of numbers is using 歸