Raju,
This is possibly because when the select was made, the count of records in the table was 14500 and when the update was done it was 15000. Check with DB admin if there was any other job running and updating the table when PI was reading the table.
One question though:
what is rownum ? is it some kind of a sequence number ?
Please look at the thread below for an alternate solution:
http://scn.sap.com/thread/3381809
Hope it helps!
Ambrish