Did you actually pick the first room? Or was it assigned the same day by DCL? That would be the difference.
Disney assigned us a stateroom at time of booking.
What the OP should say is that the PASSENGER cannot choose the stateroom number at the time of booking. DCL CAN possibly assign a stateroom at booking though.
This is what I thought it should say, but I am no expert by any means, so thanks for that clarfication.
The way the OP worded it made it sound like no staterooms are assigned at time of booking (whether by guest or Disney), and that is why I clarified with our experience that we have had our stateroom since time of booking.