The iPhone 6 has screen dimensions that aren't divisible by 8. As a result, placing a regular grid means that the last grid on the right and bottom and off.

Given that UI designs are bilateral vertically, if you want to make the side margins 16pts and aligned with the grid, the left margin will be off the grid.

To counter that you can make the center grid off by that 1pt, which normally doesn't affect the placement of objects as much.

The same could be done for the horizontal grid, however since most apps aren't bilateral horizontally this is not such a big deal (unless they are viewed in Landscape mode)