Different Rear Brake Setups - Why?

I was having a look at some bikes in the carpark today, and there's a ZX9 and an ER6F which have brackets that the rear caliper bolts to, and there was one other bike there which is the same as mine that has a tube that goes from the frame and the caliper mount has the axle going through it. Are these just different mounting systems that do the same thing or is there method to the madness of two different systems?