Previously, the Scrub to remove previously bounced addresses would remove those addresses that had bounced X times (per your My Options, Bounce Handling settings) prior, with the default being 3.
Why? Whitelisting requirements with many ISPs now dictate that definitive bounces are removed after a single bounce. As a result, JangoMail has been removing them "on the fly" during sending, as you see in Sending Complete email results, but this has not been reflected in the Scrub Group Members function until now.
A definitive bounce is one where the receiving server responds with a message indicating that the particular address does not exist on that server. Typically, these are the "user unknown," "mailbox not found," etc. types of addresses.
Concerned that a Scrub might not do what you expect? Now, get a Scrub report without actually changing any data. Use the new option at the top of the Scrub Group Members page to elect a View Only scrub.
Your Email Manager will get the Scrub Job Complete report, showing data as "VIEW ONLY." The total number of Group Members won't change, but the report's detail will show you what data would have been affected had you really run the Scrub.
Then, you can safely go back in and run the real scrub to clean your data, if you choose.
The email notification will have (VIEW-ONLY) in the Subject line, and the report will look like: