I just purchased this firearm and found that when assembled, the top lever does not fully return to center when the barrels are closed. Why is that?

All Beretta 680 series over under shotguns are designed to have the top lever rest to the right of center when the action is closed. The top lever activates a lug that locks the action and barrel together.  These lugs are conical in shape at the point where they lock into the barrel. Due to this shape, as the lug and barrel surfaces wear, the top lever will move more towards the center when the action is closed. This design gives the owner an indication of the condition that the locking system is in. When the top lever moves to the left of center, the action should checked for refitting with a oversized lug. The ability to rebuild the action with ease is one of the many benefits of owning a Beretta shotgun. We recommend that this service be performed by one of our Factory-Authorized Service Centers.