I'd bet it is for emission reasons. Having just made my GTi pass WA emission test for first time in 4 years, I learned quite a bit about hydrocarbons and carbon monoxide. Much of this has to do with mixture/fuel ratio, too, but timing does affect what comes out the exhaust.
I would suspect that US model with retarded timing (quite appropriate) is the "greener" machine.
Unfortunately, I only learned enough to get my car to pass, but not enough to explain it all. Sorry.