Apple’s ID management has been a bit of a thrash. Many of us have multiple Apple IDs — I have four that I know of (Find Your Apple ID couldn’t locate any of them). My purchases are currently distributed between two Apple IDs - one that I use for all iTunes and one that has most, but not all, of my hardware purchases.
Today I figured out how to transfer devices between two Apple IDs. It’s fairly simple. Go to your support profile and log into the Apple ID you wish to remove the product from, remove it, log into the destination Apple ID, then add it.
1. Remove from 1st profile (this is an old device, useful for test purposes)
2. Add to 2nd profile (Apple ID)
Unfortunately it would be really tedious to unite all my hardware with my iTunes Apple ID, but at least I can move the two devices on the iTunes Apple ID to another one. They’ll all be in one place — but I’ll only move things that are out of warranty lest I mess up warranty records.
I have a feeling that the initial assignment is based on a flawed matching algorithm that uses things like credit card phone and address information.