"SELECT IN SELECT" problem in MySQL

[Creative ChinaPCG]

1) the result of sql statment "SELECT rsa_lookup_id FROM `rsa_policy_plan_lookup` WHERE rsa_plan_id=1" is "11,12,16"

2) Runing in MySQL properly if the sql statment "SELECT rsa_eventNature_id, rsa_eventNature_name FROM rsa_eventnature WHERE rsa_eventNature_id IN (11,12,16)

However, if I try to combine 1 & 2 into the sql statment "SELECT rsa_eventNature_id, rsa_eventNature_name FROM rsa_eventnature WHERE rsa_eventNature_id IN (SELECT rsa_lookup_id FROM `rsa_policy_plan_lookup` WHERE rsa_plan_id=1) "

the result is not the same #2. Why??? How to solve?

Much thanks.