The first Autosizing mode sets (among others) Fixed Top Margin & Fixed Bottom Margin & Fixed Height. Basicly, you're telling the view to have the same margins to top and to bottom but not to modify its height, which is pretty impossible, so the interface builder will give priority to the bottom fixed spring.
The second case, having Fixed Top Margin & Fixed Bottom Margin & Flexible height, stretches your images on the whole content. If you haven't unchecked the view controller's option (Extend under top bars) the full content is 568(for 4inch device, which is your case) (on ios 7 if you have that option checked the viewcontroller's view's y origin is under the navigationbar & status bar , right in the very top corner).
If you uncheck the Extend under top bar option, the viewcontroller's view will begin from 64.