Correct size or over-sizing 19" tires

[Bigtop]

I am awaiting my new X5 4.8 with 19" wheels sport package. When I go to Tirerack and see upgrading tires only. When you enter in the year 2007, 4.8 sport it states that OEM is 20" wheels. Mine I am pretty sure is coming from the factory with 19" rims. Is Tirerack wrong on OEM size for an 07 X5 4.8 w/ Sport Package?

So if i just choose 19" and then when I select optional upgrade 19" tires it gives the option to put larger tires on the back (285/45-19)and smaller ones on the front (255/50-19 ). What is the advantage of doing this over having the same size on all four 19" rims. Will my car come with the same size tires on all 4 rims, or will it come with larger tires on the back rims?? I know that I could upgrade my car to come with the 20" rims that were wider rims at the back than the front, hecne the huge tires on the back with this package, however, I did not do this. So even though my rims are 19" and same width all around, can I still put the fatter tires of the back?

Sorry for my ignorance, I am just trying to learn what I can do with my new car.