I think this is the best wording of this (at least for the size class in question). I have UILabelinside a container view inside my view controller in UIStoryboard. It seems that when I add a few restrictions for UILabel, to change the font size between the size classes, the result of the execution is not my font, but what seems to be standard. As seen on the iPhone 6 simulator and iPhone 6 Plus device.
Before: Interface Builder (many restrictions):

In runtime:

Having removed the second restriction, as shown in the first snapshot, now the result is:

Things I checked:
- Fonts are in phase
Copy Bundle Resources Fonts provided by application Info.plist ,
XML- . , . , - .