The starting and ending point is that you have no recourse. The contract you agreed to specifically states that seat assignments are not guaranteed.
It is highly unlikely that you will every find out why this happened, largely because no front line agent (or even supervisors in a call center) know the true answer. Better answer to have given you was simply, "sorry, but I don't have access to that information."
As others note, the best you can do here is to keep an eye on it, either manually or through EF.