A lot of people encountered the situation that online case status was different from the actual status, e.g. approval notice is received but online status is still pending. I discovered today why this happens.
For security reason, USCIS maintains two database, one storing actual statuses while the other online statuses. The first is internal database, and no external access is allowed (i.e. any hacking is prevented). The second allows internet access (possibility of being hacked exists). Further, these two databasees do not communicate. USCIS have technicians synchnorize them, thus resulting inconsistency.
If your case has inconsstent status, do not worry. Simply call them and tell them the situation, and they usually fix the problem within 7 days.