Multiple people have been consistent reporting *extremely* limited availability in December for the last 2 bounceback offers. This suggests that rooms for that period are expected to be well booked without discounts and that WDW does not feel that they need to have a lot of offers for that time period. If there are many guests booking in anticipation of the Fall SWGE opening, we won't see a lot of availability on discounts and the discounts may be skimpy (low percentage) since discounts are offered to entice bookings. On the other hand, we won't see those discounts come out until a few months before.
If bookings aren't where WDW would like them to be, then will then offer more rooms at a discount and better percentages to fill those rooms. WDW has been much aggressive with their Spring and Summer offers, so those dates must have had softer bookings so far.