Because the Compiler takes the array base and adds the offset to it to get the value,
In the case
                  a[i] --> It is direct take the array base of "a" and move to offset i within that.
                  i[a] --> In this it is not direct but the compiler takes it in the same manner as the above statement.

The think K&R statement is right, if u r not getting this answer.

Regards
Pradeep P

It is not how much we do,
but how much love we put in the doing.

-Mother Theresa