[sirn]

I think it is a lot clearer if you compared the margin using 3GS screen sizes:

    iPhone 3GS screen resolution (480x320)
    iOS4 17+19+19+19+18px
    iOS5 17+19+20+19+17px
    
    iPhone 4 screen resolution (960x640)
    iOS4 34+38+38+38+36px
    iOS5 34+38+40+38+34px

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.

[pyrtsa]

It's not impossible to make it symmetric for both, though. With consistent spacings. E.g.

    19+18+18+18+19 (iPhone 3GS)
    38+36+36+36+38 (iPhone 4)

[sirn]

As I've said below[1], less margin on the edge will make more room for app title for the 2nd and 3rd icons.

[1]: http://news.ycombinator.com/item?id=3171496

[pyrtsa]

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.