same update query with same number of rows to be updated took 2 hrs one time and 30 min on the other

 The same update query with same number of rows to be updated took 2 hrs one time and 30 min on the other time

When I extracted awrsqrpt.sql report I found that the plan is same and the number of rows are also same the only differnece I found in the buffer gets.

There are more number of buffer gets almost 11,957,485.00 in the report when the query took 30 mints and less buffer gets 6,067,047.00 and more disk reads in the report when the query took 2 hrs.

Below is the query:

UPDATE RDW_GL_BALANCES_ETL ETL SET CCID = (SELECT GL.CCID FROM RDW_GL_CODE_COMBINATIONS GL WHERE GL.LOCATION =UPPER(ETL.LOCATION) AND GL.DEPT =LPAD(ETL.DEPT, 4, '0') AND GL.LOCAL_ACCOUNT =ETL.LOCAL_ACCOUNT AND GL.PCAT =LPAD(ETL.PCAT, 3, '0') AND GL.ACTIVITY =UPPER(LPAD(ETL.ACTIVITY, 2, '0')) AND GL.NATURAL_ACCOUNT=ETL.NATURAL_ACCOUNT AND GL.INTERCOMPANY = ETL.INTERCOMPANY) WHERE PERIOD=:B2 AND TYPE=:B1