If you read the bottom of the page when you signed up for the case update info online it says this happens when they are woking on your case. It may mean many things.
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* Note on "Last Updated" date:
Your case may have a new "Last Updated" date and you may receive an Email Notification of your case being updated, without the status of the case changing. This is due to internal USCIS processing being performed on your Case. This will be reflected in the "Last Updated" date, but may not result in a different status message.
