Why main does not return 0 here?

I was just reading

ISO/IEC 9899:201x Committee Draft — April 12, 2011

in which i found under 5.1.2.2.3 Program termination

..reaching the } that terminates the main function returns a value of 0.

it means if you don't specify any return statement in main() , and if the program runs successfully, then at the closing brace } of main will return 0.

But in the following code i don't specify any return statement, yet it does not return 0

#include<stdio.h>

int sum(int a,int b)

{

return (a + b);

}



int main()

{

    int a=10;

    int b=5;

    int ans;    

    ans=sum(a,b);

    printf("sum is %d",ans);

}

compile

gcc test.c  

./a.out

sum is 15

echo $?

9          // here it should be 0 but it shows 9 why?

Source: Tips4all