When you changed your rez you were given as different Car because the *30 was Full on the Day you wanted!
It's very unlikely that the 30 car was full. Amtrak load balances things so that each car has an equal number of rooms sold. If they booked the 30 car first, and then moved to the 31 car, you could end up with the attendant in the 30 car having all the work and the attendant in the 31 car having no work. So the routine is, book first room in the 30 car, then book the next room in the 31 car, then return to the 30 car and repeat till the train is full. The only time that doesn't happen is if someone makes a special request that alters things.