My question is this, did you check the same train, on the same day (both the day you looked and the day you want to travel) with the same itenerary? The fares can increase based on all these factors. If you look through the fare book (which is pretty thick just for a handful of routes) there will be all kinds of fares with a pretty big range among them. The fare you get can change based on booking, time of year, and many other factors, but should not change from Upper to Lower Level.