There can still be mail in the pipe for someone even after they
unsubscribe from a list. List mail travels more slowly than ordinary
mail, so the unsubscribe confirmation can get there first. It can
take up to a day or two for the pipe to drain. It might help to tell
people this when they unsubcribe.
The same thing is true of bounces, especially if mailer had to keep
trying to deliver a message for a few days before giving up. Since
those messages don't bounce for a few days, you'll still get bounces
for that many more days *after* you delete one of those subscribers.