Can you help us understand the specific setup for the Report and the separate smart list?
You should also note that the Smart list will return the # of lead with ANY bounce. If you pull the Email Performance report, it will show the total # of bounces by email, which is different. So it may be that 660 LEADS had a bounce from Sept 1-Nov 30, while 805 emails bounced in that time period.
If you are using a Lead Performance report, then it might have to do with the leads created at time frame you are using.