When a new major version of iOS is released, Apple offers it as an optional upgrade to people updating from the Settings app on their phone. The primary update shown is the minor version of the current OS.
People using iTunes or Finder will only be offered the latest version, which will be the new major version of iOS.
Here is a screenshot from Apple supporta user on Macrumours showing what this feature looks like (ignore the blue square)when it is available:
However, Apple does not offer this dual-track upgrade process indefinitely. Apple does not allow these versions to be installed anymore and you must go to the latest version that is compatible after the dual-track upgrade process is withdrawn.
Here is another screenshot from a user on Macrumours: