As we are in the era where information can theoretically be transmitted in realtime it is very common to have T/O data transmitted to the A/C after the doors are closed, and before entering the runway. This means one can accept any bit of payload that might still fit on board, and still depart on time. This is profitable most of the time, but more often than one would wish, the data-flow is interrupted somewhere, leading to an A/C lined up without having received their T/O data yet.
They might have known about the obstacle, but they might not have know whether dispatch would have it in their numbers until they received those.
Considering the number of stations possibly involved in communicating the problem, 30min are easily spent troubleshooting.
You might dispatch the flight assuming there is always the most restrictive condition, but this might be challenged by the beancounters. That's the cost of one delayed flight versus the (possibly) increased fuel burn / decreased engine life / payload left behind on the rest.
Or you offer both performance versions, and leave the captain to identify the actual condition upon departure. If payload is compromised by the more restrictive performance, verify the actual condition ahead of time, because offloading payload may lead to another delay.