select and update rownum sender jdbc

Dear all

i am using PI 7.31 sender jdbc adapter very strange work

table(TB_AA)  has 10000 row with pi_stat = 'R'   and then  sender jdbc adapter tried  10 row  in using rownum  every  120 second and update rownum

but  result of select query and udpate query  were different.  not same record update for selecting data.

co_cd,  memid  is PK.

select co_cd , mem_id  reg_n  FROM TB_AA  where PI_STAT = 'R' AND rownum <= 10

update  TB_AA  set  PI_STAT = 'S', where  PI_STAT = 'R' and rownum <= 10

query is wrong?  or  PI is bug ?