Running on this problem as well and this is my findings. This is a real mess.
There is no Medium system font on iOS7, they added it in iOS 8.2. On iOS7, after a long lag, it is picking the first font in alphabetical order (Academy Engraved).
Interestingly the iOS 7 bold system font is actually a Medium Helvetica Neue font:
(lldb) po [UIFont boldSystemFontOfSize:12] <UICTFont: 0x12c58f8b0> font-family: ".HelveticaNeueInterface-MediumP4"; font-weight: bold; font-style: normal; font-size: 12.00pt
and the systemFont is a regular Helvetica Neue.
The Workaround for iOS 7 is to pick the System Bold font in interface builder, it does look thinner when it is running on an iOS7 device than it does on interface builder. Unfortunately, on iOS8 and iOS9, it really looks bold and not medium...
I ended up switching to Helvetica-Neue Medium for those cases which unfortunately means I have a mismatch of system font/San Francisco and Helvetica-Neue in some of my screens on iOS 9. Can't wait to get the green light to drop support for iOS7.