There are a number of reasons this could happen. 

For example, a seller would bring the Final Bidding Stage forward, when several bidders are Terms Accepted and the Reserve Price has been reached during the Campaign Bidding Stage.

The transparency of the process provides real time feedback to the Seller on the price and interest in the property throughout the Campaign Stage, so when the reserve has been met they may start the Openn Negotiation early to keep the current Terms Accepted bidders engaged.