after_update doesn't fire
The after_update case called is called after the user modifies his account from my understanding of hook_user.

So that would mean that the user account has been created and the user updates his profile such as changing user image or password etc., which would trigger update and after_update.

I guess when the user is created it is only normal that the insert is triggered. If you have customized the registration so that in course of the registration the user account has been created and while completion the account information is being modified by him the update and after_update would be triggered.

If you wish to get the uid of the user who's registered most recently I suggest you could implement hook_user with insert case and query the database to get the max(uid) which would be the recently added user's id, only making sure your module has a greater weight so that user module's insert of the user happens first and your retrieval of the uid follows it. Though my suggestion feels less than optimal I believe it would work.