As is being described, "Titanforged" is not some fixed item level result, but is more a tag on item like you would have the tier of difficulty on raid gear.
That raid gear can be sourced from different raids, therefore mythic raid gear won't have a consistent item level simply because it is mythic, but mythic have some consistency to how much it is above normal for instance, in a given raid.
A random upgrade to a specific item level is not equally likely in all content due to the decreasing odds the higher the upgrade, so you should be playing in content awarding gear as near to that cap as possible for the greatest odds if that is really your aim.
Sure, the raising of the cap CAN mean the awarding of better gear from lower content, but it still remains the most likely the higher the content is to start with.