SQL & PL/SQL :: Queries Returning Same Result With Different Access Path To Table?

Following are 2 queries, which return same results as far as the input parameter is NOT NULL

select /*+ gather_plan_statistics */ * from PX_CJQ where decode(:x,null,1,object_id) = NVL(:x,object_id);
select /*+ gather_plan_statistics */ * from PX_CJQ where object_id = NVL(:x,object_id);

However the execution plan differs a lot The FTS cost or rows accessed count also varies what could be the reason? The PX_CJQ is simply select * from dba_objects and has index on object_id which anyway is not being used in this case

variable x number
exec :x:= 28
Query - 1
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SQL> >select /*+ gather_plan_statistics */ * from PX_CJQ where decode(:x,null,1,object_id) = NVL(:x,object_id);
SQL> >select * from table(dbms_xplan.display_cursor(NULL,NULL,'ALLSTATS LAST'));

[code]...

Query - 2
***************
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SQL> >select /*+ gather_plan_statistics */ * from PX_CJQ where object_id = NVL(:x,object_id);
SQL> >select * from table(dbms_xplan.display_cursor(NULL,NULL,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT

[code]...

29 rows selected.
SQL> >