You must consider the drivers at least. One likes understeer, other oversteer. One likes to setup the car for the fast parts of the track, while other likes to be fast in corners ( compromising top speed inevitably). So there are other compromises that you have to consider. And all this things have it's effects on suspension setup. It's quite possible that the ideal tyre performance sweet-spot is not the same sweet spot for tyre degradation too. So it's a difficult and sensitive balance of a parameters that teams could find, but it's not as straight-forward as it seems. Like we saw at Monza 2010, when McLaren couldn't decide over two rear wing configurations. With this year's "sensitive" Pirelli tyres, every change in car setup could have an effect on tyre life/performance. Not to mention the fact, that almost every race is a new experience for tyre specialists.
Car differences like suspension geometry is another thing to consider. We saw Sauber mastering the tyre life, but not performance. We saw Merc overcooking rear tyres...etc...Maybe next year it would settle down, but now everyone's just experimenting quite a lot IMHO.