It doesn't makes sense to me! I guess I'd have to hear it. I did read recently that somewhere in the UK
John is pronounced with a long O, so I thought maybe that was it.
John and
Jon are the same here. I actually would think the opposite of how you describe it-- in the UK a longer, open sound is used. If I try to think of
John said with an English accent, it sounds like JAWN. But in my accent,
John is more horizontal-- like JAHN.