A Query Sentence Return A Puzzle Result

the talbe row like this:

  ID   Name   Scoe
 11     Tome    20
 12    Jack   30 
11     Tome    40
 12   Jack   10
 13   John    10 

My query command like this:

Select T1.Id,T1.Name,T2.math
from st T1
right  join
(Select Id as Id2,Sum(Math) as Math from St  group by id) T2
on T1.id=t2.id2
where t1.id = t2.id2

While the reuslt is :

Id      Name   Score

11
    Tom

       60

11
    Tom

       60

12
    Jake

       40

12
    Jack

       40

13
    John
       
 10
   


 


I am wonder :the T1 gives a table with six rows, the T2 gives a table with three rows,  and I use RIGHT JOIN to connect the two table,the result should be a table with only three rows.I tried INNER JOIN, the result is same.


but why ? please help me !