The mystery is part of an elaborate scheme for a means to an end. The end being, what method of design would help keep players playing the game (thus extending their subscription).

In the case of ambiguous numbers without posting exact information.

Lets say Player already has item.i140 (Stat A and B)

SE implements new items, ilvl 150, Player sees an item item.i150 (Stats C and D)

Ideal Reality: In the case where exact numbers were posted, item.i140, ends up having greater dps/hps than, item.i150

Reality: However in the case where stats are left in a shroud of mystery, many players end up looking directly at the main stat boost along with the ilvl, as a means of whether to spend the time to upgrade, whether its content farming or tome farming. Thus creating a time sink.