It is possible the 48 was totally booked, so they sold all the 448 roomettes, but later some of them were cancelled or still available between ALB and BOS, freeing up some of them but not for the full trip, or even that when they originally opened up 48/448 for reservations on your travel date last April or May, they offered both 48 and 448 to travelers between Chicago and destinations west of Albany and enough people booked those rooms that by the time you made your reservation there were no through roomettes all the way from CHI to BOS. The best strategy would be to swap rooms with someone in 448 who wasn't going past ALB (maybe their destination is west of ALB or they are transferring to a room or to coach in 48 for the last leg to NYP?) Amtrak doesn't seem to like reassigning rooms arbitrarily like this, though they do it if a car has been cancelled and sometimes for no apparent reason.
People might have chosen 448 for some particular reason other than destination, though I don't know what that might be. Maybe 448(sleeper)/48(coach) was cheaper, or maybe 48(sleeper)/48(coach) was also offered, and they didn't understand the difference. I found making reservations on the split trains confusing the first couple of times I made them, and the first time, I booked myself in a roomette on 449, switching to a roomette on 49 in Albany, not realizing I could have ridden 449 all the way. (On the return trip, I got a 448 roomette all the way, but that was mostly by chance.)