Before iOS5, they were trying to maintain consistent spacing between icons while in iOS5 they changed the home screen margin to be asymmetric while maintaining the same margin ratio between non-Retina and Retina displays. They can't use 35+38+38+38+35 because that would leave non-Retina at 17.5 pixels for left and right margin.
Oh true, missed that. Finally pulled out the phone to realize this. And then the other option 16+20+20+20+16 / 32+40+40+40+32 already has too big a difference between the margins on the edges compared to the ones between the icons. Makes perfect sense now.
