I don't think it counts when you have different models, like the 5c or another small-screened model that will replace the c (I assume they'll give up on that, or else redesign it significantly).
There are two (well, now three) iMac models to choose from, ignoring the different CPUs, RAM etc. There are also a range of MacBook models, including the Air.
So why not a number of different iPhones? It was rumoured for some time before Apple actually introduced the 5c, and eventually happened. Now there are the two 6 models.
The question is why Apple took so long, but clearly Steve Jobs knew best and figured one single model was all that was required and people would want whatever they were given. He was right, to a degree.