Honestly, the mechanics of how it all works are probably only known to a couple product managers and business stakeholders at the third-party that runs it. I'd go as far as to say it's probably proprietary and not even NCL knows the exact business logic. NCL just contracts with this company to fill empty cabins at reduced rates closer to the sail date and there's probably certain controls NCL has, but it's otherwise a grey box system with some kind of contractual obligation for certain performance metrics.
If there is a specific cabin type you're trying to get, I'd recommend having that be your 'best' bid of the lot....but it generally depends on the availability and demand more than anything.
It's a big number crunch, basically. NCL obviously wants to maximize their capacity aboard the ship and getting the 'potential upgrade' numbers early lets them adjust pricing on the lower level cabins as necessary. If they're at 50% capacity 3 months out, knowing their capacity to top fill the higher level cabins helps them get a better idea of A. what sorts of thresholds they want to place for those cabins and B. how many of the vacated cabins will be available closer to sailing.
So to get a short answer, they've calculated that there is a point of criticality in each sailing where opening up the bidding lets them forecast the departure day supply and demand ratio.