How Do You Make A Multiplication Chart On Excel In A Multiplication Chart For Any Base, Why Does The Last Line Answers Add Up To One Less Than The Base?

In a multiplication chart for any base, why does the last line answers add up to one less than the base? - how do you make a multiplication chart on excel

Base 6

X 0 1 2 3 4 5
0 0 0 0 0 0 0
1 0 1 2 3 4 5
2 0 2 4 10 12 14
3 0 3 10 13 20 23
4 0 4 12 20 24 32
5 0 5 14 23 32 41

If you add 1 +4 = 5 2 +3 = 5, 3 +2 = 5 Why?