Why in the multiplication table, different characters such as 如，得，歸，中 are used?
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