How did my headset get this way?
Nathan's comment is solid -
Assuming Stephan removed the keyed washer when he took off the lock nut...

How did it get this way?
It's hard to say, here are some possibilities.  


It may have been cross threaded from the factory. Seems like a long way to cross thread something - all the way from the top of the steerer tube to it's current   location and then back up to almost the top. Cross threading usually results in some thread damage that should be visible in the steerer tube threads.  
A previous owner may have cross threaded it. Same observations as option 1.  
I can't think of anything besides cross threading (and Nathan's suggestion)  


How to fix it?
First, check Nathan's thought - look for a keyed washer sitting on top of the race nut. It's labeled "Washer" in the diagram below.  




Assuming cross thread - since you are 1/8 of an inch from the top with a rounded nut I'd look for a tool that will like a large vice grip to clamp on to turn in the last little bit. Some penetrating lubrication might help.
I'm worried about cutting the race off. It's hard to imagine how you can cut it without damaging the steerer tube threads. If you use some kind of a small cut off saw, don't go all the way through, make a couple cuts and experiment with a chisel to remove a section of the race it might not damage the threads.
I've read of people having success using heat to remove stuck nuts. I've never had any luck with it.


Maybe someone else will have a better suggestion.

If the threads on the steerer tube are OK all you'll need is a new adjustable race.
If not, you may be able to clean up the threads with the right threading die.
If not, you'll need a new fork