The versions of the CD-ROM are not simply additions of new publications. They also change 'old light' into 'new light' sometimes as well.
For example, in a printed version of a 1980's magazine or book, you may find the phrase such as "Armageddon will occur within the 20th Century". Early CDs may contain this but later CDs may be changed to "Armageddon will happen soon" since it was clearly incorrect.
It is simple changes, just a few words here and there. They don't change or remove huge articles as this would be too obvious to Witnesses using it.
By changing little things, JWs think that it is an exact copy of what was originally printed and forget what was really in print.