constant on LHS is ignored

[aufflick]

let a = scrollView.right == view.right - 100
let b = scrollView.right + 100 == view.right

would appear to be equivalent, no compile or runtime error occurs, but in the second form, the constant is ignored:

a: <NSLayoutConstraint:0x7fcd02dee830 ETVTileHostingScrollView:0x7fcd0221b000.right == UIView:0x7fcd0141d840.right - 100>
b: <NSLayoutConstraint:0x7fcd02ce4be0 ETVTileHostingScrollView:0x7fcd0221b000.right == UIView:0x7fcd0141d840.right>