What he said. The Unabridged series of RS books is often abridged, removing aero units, support units, and sometimes even battle armor to speed up the process and get the tanks and 'mechs available to players. I don't recall any specific official statement, but I believe their desire is to go back and add the removed stuff at some point, probably once the RS-making process becomes a bit more streamlined. If you buy a RS product right now, you'll probably be allowed to re-download the truly unabridged one for free when it becomes available.
(Note: All this is guesstimation from what I've seen on these forums and heard in Battlechats. Don't count on me to have hard data, especially since my mind at best resembles not a steel trap, but a steel colander.)