No, it's not an unreasonable conclusion at all. And in fact many years ago I too believed that a bucket what attached to each room. But then several trusted friends who work/worked for Amtrak debased me of that notion. It's also not tied to what level the room is on either.
For example, last I knew the current booking order for rooms was 2, 3, 4, 11, & 12.
After that, I'm not sure where they went next. Now, if for example you were booking the Texas Eagle with only 1 sleeper and there were 4 rooms placed in the low bucket by revenue management. Then rooms 2, 3, 4, & 11 would all go for low bucket. Room #12 however would sell at the next bucket level. If there were 5 rooms in the low bucket, then of course room #12 would also sell at the low bucket price.
There are 5 bucket levels in total. Revenue management (RM) initially picks how many rooms are placed into each bucket. And there is no requirement that each level actually have a number placed into the bucket. In other words, it is possible for RM to put 5 rooms in low bucket, skip the next bucket, put 2 in the mid-level, 5 in the next highest, and the remaining in the highest bucket. Additionally, nothing is set in stone either. RM or the computer can move the quantities of available rooms left around at will as they please. Well the computer of course follows certain rules, but a RM manager can do what they think best. So for example, around Thanksgiving I wouldn't bet on any rooms being placed in the low bucket and a very low number in the next bucket.
All of that said, a regular agent can if they know what they're doing, essentially swap a room for anyone at the same bucket level if they know what they're doing. Upper level, lower level, current bucket level for selling rooms are all irrelevant if the agent knows what to do and how to override things. Another trap that agents fall into is forgetting how to request a specific room. Under normal circumstances, an agent pretty much does what Amtrak.com does when making a reservation, they just request the next available room and ARROW provides the next room based upon the pre-approved order of rooms being sold in the car. And note that with more than 1 sleeper, ARROW also spreads rooms out across the available cars. It doesn't sell all the rooms in the first car, before moving on to the second car.
But agents do have a way to bring up all the available rooms and to pick a specific room within a specific car. The problem is that too many forget how to do this, or think it's too much work, or what ever. So instead what they do is request next available room. If that's the room that the person wanted, great! If not, they hold that room and request the next available room. And they repeat that until they get what the person requested. This method has too problems, first, you start running through all the buckets because you're holding all those rooms open. Second, the RM computer sensing a run on that train starts moving more rooms into higher buckets because it thinks sales are going through the roof and it wants to maximize revenue. It doesn't realize that an agent is causing this issue and when they're done, the agent is going to release all those rooms back into inventory, save the one that they really wanted.
I hope that this helps to explain things a bit.