Arriving early to a hotel, I often hear "your room is not ready yet."

Some hotels do reserve rooms for specific requests i.e. disabled or family, others are as you say are a room but not defined, if the previous customer is late checking out then the timeframe set aside to refresh the room is shorter so you may get a delay. 

room is not ready (dirty) coz guest has came before chek-in time ... or if you have any other option in room tell them...conditionally... or you greet them reply to them sorry for the inconveniece and plz have seat in lobby or till that time we have restaurent n bar...