[Closed] Avid Back n Front

For some reason I have only just noticed that my bike has different front and rear brake pads. Its a Canyon Nerve XC 9 from 2012 and has Avid XO brakes. The listing states from is 180, rear 160 but I recently had to change my rear and found that the spare pads I have been carrying round for years dont fit- only fitting the front. So I was suprised that Avid make a front/ rear combo with different pads. The rear has a curved pad and has twin pistons, but the front has a single piston and is a smaller squarer pad

I presume the curved pad is to compensate for the tighter curve of the smaller rotor but why the twin piston on the back not the front? Presumably the smaller rotor has less stopping power, but the twin piston and wider pad will compensate for that, and it might even be possible this is the first time I have had to change the rear pads since buyng the bike, even after an alpine trip last year! So it does seem to improve longevity of the rear pad, just found this all an odd setup

When looking at replacement pads it seems my front is listed as Elixir and my rear as Avid XO