Variables in Subquery gives wrong result

CREATE TABLE test (Id INT);

# Insert some rows

INSERT INTO test VALUES(1);

INSERT INTO test VALUES(2);

INSERT INTO test VALUES(3);

# The Following query is ok:

SELECT

   SUM(VariableA),

   SUM(VariableB)

FROM (

   SELECT 

      (SELECT @varA := 20) AS VariableA,

      @varA AS VariableB

   FROM test) as A;

+----------------+----------------+

| SUM(VariableA) | SUM(VariableB) |

+----------------+----------------+

|             60 |             60 |

+----------------+----------------+

# The same query, but remove SUM(VariableA) in first select. Should still give 60 for VariableB, but it returns NULL.

SELECT

   SUM(VariableB)

FROM (

   SELECT 

      (SELECT @varA := 20) AS VariableA,

      @varA AS VariableB

   FROM test) as A;

+----------------+

| SUM(VariableB) |

+----------------+

|           NULL |

+----------------+