I dont think they could be that sloppy and think of the chaos it would cause at the airport.
What probably happens is that the tour guide allocates the rooms on day of arrival before coming to meet you., or on the bus awaiting your arrival, she can probably swap some over before you reach the hotel, so be nice to the tour guide on arrival. If you are not happy with your room, go straight back down to reception and complain loudly, chances are the guide will still be there.