Hi, choronr -
You assume a ... regimented approach?
Seriously, manzanita is a pretty diverse plant. I guess all the models are meant to be in sort of an adult stage of growth, and reflect the diversity of the appearance one would expect in a clump. I can't remember who it was that really specifically stated it as such (here in these forums), but if you really want to get a realistic look for a population of plants, you need more than one model. Otherwise, you end up with variations of the 'christmas tree farm' effect (to quote said unknown critic).
Manzanita ranges in adult size from a ground-hugging shrub a foot or two high, to a small tree form 20 feet or so tall. The height and form depend largely on the elevation. In the Sierra foothills, it's a small tree. In the peaks, it's a shrub that ranges up to 12,000 feet or so.