Using AmSnag, took a quick look at CZ Roomette fares from now through Oct 2017 - 11 months worth - looking for the frequency of low bucket ($311) Roomettes and found this:
Dec-11, Jan-12, Feb-28, Mar-26, Apr-28, May-25, Jun through Oct-0. Those that weren't low bucket were a variety of the other buckets from $424 to $758. AmtrakBlue seems to have nailed the situation, at least for the CZ. Don't know if this same pattern (no low bucket Roomettes beyond 6 months in the future) holds for other routes.
However, this one-time look shouldn't dissuade anyone from booking beyond 6 months out - especially if your travel date is set in concrete: book the sleeper you want (just for the sake of getting it) even if it's not low bucket; create an AmSnag Alert; when the price drops for the day you booked, call an agent and have your ticket modified to the lower fare. If you travel date is flexible, you can wait and hope a low bucket appears in your travel window - hoping they're not all sold out before you act.
Edit: Got bored and looked at three other LD trains:
• The Empire Builder followed the same general pattern as the CZ (132 low bucket Roomettes available during the next 6 months, but only 2 low bucket Roomettes available from Jun through Oct).
• The Crescent was like the Empire Builder (94 low bucket Roomettes available during the next 6 months, but only 2 available from Jun through Oct). Also noticed many 4 day periods blocked out during the week in Jan & Feb, probably due to track work.
• The Silver Meteor was just the opposite (only 23 low bucket Roomettes available during the next six months, but 117 available during the Jun through Oct 2017 period).
All this sort of backs up my rule of thumb for Amtrak fares - you can't always sometimes tell what you least expect to happen the most!