IOS app Nav bar back button

I have looked at all the navigation bars on all the story board scenes of my app, and I have gone through all the view controller m. files and there is no mention anywhere of the navigation bars settings. The mystery is that I have two different pages, one where the back button is in lower case (back), another where it is in title case (Back), and other pages just have the cursor image < or nothing at all. If I click on the storyboard navigation bar area, it highlights in blue, and there nothing obviously there.

I cannot find where this has been defined !! Any ideas?