I dont understand why the setup on my car is a 225/40/18 for my fronts and a 255/35/18 on the rear. I do understand the staggering of width but the side wall is what confuses me, why a 40 on front and a 35 on the rear? would there be any consequences for making them match, why is it preferred to change the differences in sidewalls?

I am planning to run a 265 on the rears with my next tire change and looks like I might have to get the fenders rolled too..

 

The number for sidewall height is a percentage of the width in mm, not a fixed number. Even though the sidewall numbers are different, the tire diameter front to rear is the same. 40% of 225 mm is roughly equal to 35% of 255 mm.