This is because the update checker:
1) only checks once a week, not every hour or day...
2) only checks a while after you've been online (it doesn't check immediatly once you're signed in)...
3) the returned check from the server also need to be updated with the latest version. And that is only done a while after its release to avoid millions of people downloading the update at once.
So, there is nothing out of the ordinary, just be patient... Plus! will warn you...
Or you could always manually download and install it, there would be no difference at all; the auto-update will download exactly the same file you would download manually. It also will execute the file exactly like you would manually execute it...
/ 