That can happen but the upgrade options at that price point might go pretty quickly. Possible the J cabin is not that full so the "low" bid and upgrade currently match.
The bid amounts are I believe set at the outset based on route and tend not to change (I'm open to correction here).
If you are looking to upgrade I'd secure it with the offer you see currently as I don't think the bid offer will drop below the min you see now but the direct upgrade price will almost certainly.

Plus you'll get the avios and tier points (if that interests you) with the purchased upgrade direct.