Event column null in DBA_HIST_ACTIVE_SESS_HISTORY

Why is the event NULL below ? The SQL in both the cases is a SELECT statement.

SQL> select event,count(*) from dba_hist_active_sess_history where snap_id between 87499 and 87500 and sql_id='85y6w40gzwvyf' group by event;

EVENT                                                              COUNT(*)
---------------------------------------------------------------- ----------
                                                                         14 <<<<<-----
db file scattered read                                                   38

                                Gets              CPU     Elapsed
  Buffer Gets   Executions    per Exec   %Total Time (s)  Time (s)    SQL Id
-------------- ------------ ------------ ------ -------- --------- -------------
     1,095,121            1  1,095,121.0    2.3    52.02    555.57 85y6w40gzwvyf
Module: idp1007@bupaiap3 (TNS V1-V3)
select to_char(sysdate ,'YYYY-MM-DD:HH24:MI:SS') ,CLA.HOUSE ,CLA.CUST ,:b0 ,:b1
,930 ,CT.CUR_BUCKET_FLG ,2 ,to_char(CLA.CYCLE_DATE,'YYYY-MM-DD') ,to_char(CLA.LD