Probably a best practice issue. The rolling diameter seems to be very close for the two sizes you specd, and also very close to the OEM spec (with the difference being in width). My opinion is that there's enough space in the wheelwells to accomodate wider tires, though I'm less certain with wheel sizing.
You could play around with a tire and wheel size calculator here:
http://www.e90post.com/forums/forumdisplay.php?f=12
*Disclaimer*I am not a wheel and tire fanatic, so what I write is ~ opinion/conjecture.