When someone books a seat or room, and later cancels it, that seat or room re-enters the inventory AT THE ORIGINAL BUCKET THAT IT WAS BOOKED AT!
PUTTING SOMETHING IN BOLD AND ALL CAPS DOESN'T MAKE IT TRUE. Your understanding of the bucket pricing system is flawed.
My actual guess is that the room is moved back into inventory and repriced based on the bucket value of the then remaining inventory quantity. That could result in a one bucket drop in price.
That is, more or less, what happens.
So how else do you explain 4 bedrooms left costing less than 3 bedrooms left?
Assuming you just said that backwards (and meant to say '3 bedrooms left costing less than 4 bedrooms left'), one explanation is that the number of rooms available at each bucket can be different for different dates, different directions, depending on anticipated demand.
I'm not sure where the "canceled space goes back at the original bucket paid" myth comes from, but it isn't true.
The space available in each bucket (and this holds true for both coach and sleeper sales) is set as a percentage of overall capacity for the train (and, contrary to another popular myth, rooms are not preassigned specific buckets; the reason changing a room can increase your price is only slightly more complicated to explain). When the capacity and percentages are set, Arrow will translate that into a specific number of seats/rooms.
In a hypothetical example for a given train/date, let's say there are 10 rooms for sale on a train. The buckets might be S (highest price)-100%, A-80%, B-60%, C-40%, D-20%. In simplified terms this would mean that you would expect to sell 2 rooms at each bucket. In a more detailed sense, Arrow is selling the D bucket until 20% of the space is sold, the C until 40%, etc.
There's no guarantee that every bucket will have space available. D might be set to 0%. Heck, everything but S might be set to 0%. That can also change between the time space goes on sale, and the time the train departs. Revenue management can adjust the percentages as deemed necessary to maximize potential revenue.
In the above example, the first person to book a room would get the D rate. If the other nine rooms are sold, and then person #1 cancels his space, the room will then sell for the S bucket, because 90% of the space is sold (which exceeds A's 80%). If eight rooms were sold when person #1 canceled, then 70% of the space would be sold, which means that an A bucket room would become available.
The bucket level is determined by the lowest overall availability between your start/end place. So if a train routes A-B-C, and is 80% sold A-B, and 50% sold B-C, a passenger booking A-B or A-C would see S bucket availability. A passenger booking B-C would see B bucket availability.
Bucket pricing in a nutshell.
Now, why does the price (sometimes) change when you want to change rooms?
If you have already booked your space, any change to your reservation means you're now subject to whatever the new price is. Changing your room is seen by Arrow as changing your reservation (because the agent has to book your new room, and cancel your old room). Therefore, the system will want to charge you whatever the current bucket is.
If you're making a new reservation, the agent will have to specify, in advance, what room is desired (as opposed to letting the computer auto-assign the room). If they let the computer pick the room, then try and change it, well...see above paragraph. Since there are usually only one or two rooms available for sale at any given bucket to being with, if you happen to get the last room at that bucket, and the agent then tries to change the room, well, the next room available would be at the higher bucket, regardless of what room number is ultimately selected.
One way around this is to cancel the room that the computer auto-assigned, book the specific room desired, and hope that, in the mean time, nobody else has booked space on the train bumping up the bucket for the sleepers. The other way is for the agent to book the second room and do a price adjustment to the price of the first room, then cancel the original space.