They are only guaranteeing a room that would accomodate 2 adults. The huge majority of hotels have options (1 king, 2 queens, etc)
For a trip to WA D.C., I got a room, and needed a larger one. I got the rm for $66 per night, and when I got there, I had to pay a $30 per night upgrade to accomodate us... the rm ww got was normally $270 per night - it was actually an apartment/suite - very nice. I wouldn't hesitate to do it again, but I will always go for the high stars.