You are right. The grey boards are games not yet started. Tournaments are set up so that everybody places everyone else in the group twice once with white and the other with black.
Some tournament directors decide that both games will be played at the same time - hence no grey squares as all the games would have started.
In your case, the tournament director decided that only one game would be played at at time and not both. Once the first game is over, the new game will start and the the grey board will become blue.
For example, in the tournament below the 1's and 0#s are completed games, the blue boards are games in progress. What are the grey boards? If they are games not yet started why haven't they started?