the oracle snapshot log question

1. there is a table emp,now we will create snapshot log on it:
SQL>create snapshot log on emp;

2. 
SQL>create snapshot s1_emp as select * from emp

3.
SQL>create snapshot log on s1_emp

4.
SQL>create snapshot s2_emp as select * from s1_emp

5.
SQL>alter snapshot s1_emp refresh fast start with sysdate+1/86400 next sysdate + 1/86400;
SQL>alter snapshot s2_emp refresh fast start with sysdate+1/86400 next sysdate + 1/86400;

6. the all process is emp ---snapshot---> s1_emp ---snapshot---> s2_emp,the refresh is 1s.

7. now we will update the table emp
SQL>update emp set ename='kxf' where empno=7934;

8. the result is the snapshot s1_emp hava bean updated,but the snapshot s2_emp not, why?