I thought this was a bug after the patch went live when I saw my mythic Somber Gaze set had an unfitting helm, the heroic version Rune-Enscribed Hood.
Apparantly this is intended and give no reason to do why did it.
So if anyone comes here wondered the same thing, this is your life now
T17 Heroic set
T17 Mythic set
edit: It's tier 17 not 18