An hour is normal I think, RSVPs don't get returned to the "pool" immediately - probably some sort of batch release back to the system.
A few months back I had actually started the process to reserve a time and then closed the web page at the last moment as my family members and I discussed the itinerary over the phone. After 5 minutes to resolve the dispute I went back to reserve it and the time was gone!

What're the odds that somebody was looking at the exact same restaurant at the exact same time and trying to score a late lunch reservation at 10pm at night?! (it wasn't even a popular restaurant!)
It came back an hour later. But it was a tense 60 minutes.