This could be deliberate. Perhaps the 4830 was just a little too good and there was a fear of cannabalising higher performance part sales.
Even with 560 shaders, the performance is great. And they needed a gap between 4830 and 4850 and 4870 to warrant pricing in "different market segments".
I'm sure they werent expecting a genius like w1zzard to spot that so early on.
There is no way this was "a mistake". A GPU manufacturer isnt going to do a simple math mistake about the number of shaders on/off.