It will happen everytime in bigger quotas where running RAC is less than 10 or even less than 100, sometimes even WL 100. Smaller quotas it may not happen. Infact, if I have to book for family and there are few berths available, I never book ticket. I keep on checking until the status comes in RAC, and then book. The advantage is that I get compact accommodation with desired berth types. I do this for bigger quotas and this has worked always.