Database Updates on same data with varying buffer get across different executions

Hi ,

  I am noticing a  strange situation on Updates running on the database . An Update statement running on a table with Full Table scan has 4 times more Buffer gets on one run wherease less buffergets on another run . However the table row count and the now of rows being updated are same in both the runs .

As part of the job , The table is truncated and Indexes are dropped daily and reloaded and the indexes are recreated followed by stats gathering .
Nearly 5000+ Updates are run on the table where majority of them are index scans . However there are few updates which use full tablescan and are exhibting this strange behaviour .