We've been trained to be like this. Battered Child syndrome.
However, I'm more interested in how GW operates when it comes to new editions releases. If the rumors of new models sitting in a warehouse are true, along with the statement that "no ink had been lain" on a new Eldar dex, then this means that the studio basically comes up with the models, and the game designers have to come up with rules to fit it in the book.
This really does smack of "opposite" of the way its should work from a game balance perspective. However, since GW has said that they're a "model" company not a "game" company (which is pretty rich, considering its name) then this would make more sense.
I mean, it would explain the Dreadknight :P