Why wouldn't they upgrade if they don't sell out the seats?
There is also a order of who get upgraded first. It makes a difference which status the passenger has, and also the status of the account whose plus points are being used. So even if the points are coming from a global account, if the passenger has lower status than someone else, the global to higher status will be first...
Also, anyone in premium economy trying to upgrade to biz, will be higher on the list before paid coach tix, even if the statuses are higher...