I'm guessing as you join the queue the system sets some initial ELO difference tolerance, meaning that if you are, say, 1100 rated, it will try to not match you with a guy that has less than, say, 1050 or more than 1150 ELO, and as the time passes it increases that tolerance with the assumption that a bad game(against someone far better or worse) is better than no game.
Could be wrong though
Could be wrong though