Like you said, that's purely speculation on your part. I never assume. The Mirage factory tire is on the open market in the States Almost every tire vendor seems to carry the factory Falken tire found on the Mirage.
I would like to think putting a different UTQG rating on the same tires would be illegal, but I don't think those ratings are regulated by anyone.
Ask Falken if their 320BB rated tire with no warranty is the exact same tire as their 720AB rated tire with an 80,000 mile warranty & let us know what you find out?