Resource Manager questions

I'm using db version 11.1.0.7 on OS HP UX itanium.
I've create my resource manager plan and i start it with command

exec dbms_resource_manager.switch_plan( plan_name => 'GBS_LIMITS_PLAN' );
exec dbms_resource_manager.switch_plan( plan_name => 'GBS_LIMITS_PLAN', allow_scheduler_plan_switches => FALSE );

but some time, automatically resource manager switch to DEFAULT_MAINTENANCE_PLAN.

In DBA_SCHEDULER_WINDOWS i find this information

SQL> select window_name,RESOURCE_PLAN  ,
  2  from DBA_SCHEDULER_WINDOWS
  3  /

WINDOW_NAME                    RESOURCE_PLAN            
------------------------------ --------------------------
WEEKNIGHT_WINDOW               DEFAULT_MAINTENANCE_PLAN
WEEKEND_WINDOW                 DEFAULT_MAINTENANCE_PLAN
MONDAY_WINDOW                  DEFAULT_MAINTENANCE_PLAN
TUESDAY_WINDOW                 DEFAULT_MAINTENANCE_PLAN