When a deleted answer is edited, that bumps the thread to the front page and to the active questions list. When a deleted answer is undeleted, that does not bump the thread.
Bumping a thread due to a deleted answer being edited serves no purpose. For the large majority of users, this is invisible activity: the bump is just noise, it's a waste of time to open the question and see that nothing has changed. For users who can see deleted posts (10k rep privilege), this is visible activity, but not actionable. If the edit is good, the editor can flag the post for undeletion. If the edit is bad, there's nothing to do, the answer is already deleted anyway. So we only end up with people wasting their time.
(Having wasted my time in this way, I decided to waste more of it by writing this feature request.)
True, there is the case of new users who posted a non-answer which was deleted by a moderator, and who proceed to edit their answer into something worth having. But there's no guarantee that a 10k user will notice the edited answer and will flag it for undeletion, and anyway there's a better solution for that, which is to encourage the user to repost (or let him flag his own post).
On the other hand, undeleting an answer is visible activity that should be reviewed. The way bumping works now allows bad content to fly under most radars: post an innocuous answer, delete it, edit it (it gets bumped but only 10kers will be able to see the content, so hope they don't notice), and finally undelete it (no bumping now). Good content can equally get unnoticed: someone posts an answer, realizes it's wrong, deletes it, then later edits it and undeletes, but the bump will happen too late (though in this case it's usually not much too late).
Now undeleting an answer should not indiscriminately bump the thread, because that would allow any answerer to keep bumping by deleting and undeleting his answer. There's an obvious solution, which goes in hand with not bumping after an edit: bump on undeletion, but only if the answer has been edited.
For self-deleted answers, this accomplishes the objective that the edited answer will be noticed, better than the current behavior. For non-self-deleted answers, this accomplishes the objective that such answers are good and buried, better than the current behavior.
(It would be nice, but presumably a lot harder, to unbump a question if the answer that caused its bump has been deleted. That this allows edits to be unnoticed is only due to the current behavior of undeletion not causing a bump, which my proposal would fix.)