Isn't it just a matter of production logistics? HM for the Americas, Vitra for Europe?
Presumably, they're made to the same standards, even though there are small differences (like the base of the Lounge chair).
What's most irksome is the price difference. I would love to get a HM Lobby chair at the listed price, but I have a hard time justifying paying more than twice as much for the Vitra version :-(