I actually think they release additional times gradually over time, even when park hours aren't changed. I have had many experienced when there were no reservations available during certain time periods for weeks and then suddenly there are all kind of times available for what I initially wanted. I have found this happen very frequently at BoG in particular. Plus people cancel reservations all the time as their itinerary changes. I tend to speed way to much time "stalking" the dining reservation system for that very reason and I have NEVER not gotten an ADR I wanted within a 30 minutes of when I initially wanted it.