There's no font on iOS called "Avenir Next Condensed", there are however fonts named "AvenirNextCondensed-Italic" and "AvenirNextCondensed-UltraLight" amongst others. When you specify a font that doesn't exist it tries to fuzzy match against the fonts it has but it doesn't always get it right. Easy fix is just to specify the correct font: "AvenirNextCondensed-UltraLight" in this case.
There's a list of fonts available on iOS here: http://iosfonts.com/