I believe that at some moment iPhone and iOS tags were synonimized.
And later on I believe there were broken down: Break the [ios] -> [iphone] synonym
However, I think they should be synonyms.
Yes. In the ideal world, where everybody put correct tags on the question, people should use iOS to ask generic question about development for iOS and iPhone to ask some questions particular to iPhone (as example UI design questions).
However, reality is that people use randomly (and interchangeably) ios and iphone tag. And taking number of questions (120k and 46k) there is no reasonably way to separate them out.
Should these several tags become synonyms again?