The "usual" procedure at T-Rex is to have the outside desk open to check in for reservations, and walk-ups would wait at the inside desk. The outer desk is not always open, especially during slower times. It's possible that things get unexpectedly busy and without the outside desk, reservation-holders get stuck waiting in line with walk-ups.
I had this happen to me last September, and a manager explained what had happened. At the time, she said they would make an effort to ask for reservation-holders if there was only one line and it got long. Let's hope this happens - otherwise, what's the point of having a reservation, even if it is only a priority seating?