iOS 8 adds the concept of “content margins” to UIView. The “What’s New in Cocoa Touch” video from WWDC 2014 discusses content margins (under the title “Adaptive Margins”) starting at about 12m30s.
The default margins are 8 points on each side. Each end of a layout constraint can be relative to a view’s margin instead of to the view’s true edge (use File > Open Quickly to go to the definition of NSLayoutAttribute to see the possibilities).
Your constraint is “relative to margin”. When you create the constraint with the “Add New Constraints” popover, you can choose whether it’s margin-relative or not:
It always defaults to margin-relative; you have to turn the checkbox off every time you add constraints if you don’t want them to be margin-relative.
You can’t change whether a constraint is margin-relative in the quick-edit popover of your screen shot. Instead, double-click the constraint to bring up its full Attributes inspector. There, you can use the popup menus to select, for each end of the constraint, whether it’s margin-relative or not:
