Originally posted by mb there may be a possibility that older data maybe moved to an archive table.
then you will have deletes. If a block gets down to 79% full, it will get put on the free list. If you are deleting a lot of data, the chances are good that you will get many blocks that are 79% full. These blocks then get put on the free list. When you go back to inserting data, Oracle will seach all these blocks only to use another 12% (91-79) of the block. This would yield good utilization of the block, but slower insert performance.
If the PCTUSED was set to a smaller number, say 10, then the block would have to get down to 9% before it was put back on the free list. When the block finally got back on the free list, you would have 82% (91-9) use of the block before it goes off the freelist again. Since this would leave unusable space in the block until you get down to 9% you would "waste" space but your inserts would go much faster because they would not have to search an artificially long freelist.