The incredible shrinking forks or the incredible expanding headset!?

The cup screws on fine, It indeed appears to be just the lock ring.

What's the odds on both the cup and fork shrinking, and to exactly the same size?

It's either a freak event - one in 1,000,000,000,043 or..

• your locknut is the wrong one
  
• you stripped the locknut threads when removing it
  
• locknut never fitted properly in the first place and was just held on by rust/dirt.
  
• the portion of thread on the steerer that the locknut screws on is worn and now you can't tighten it properly, but the rest of the thread is fine so cup is fine.
  
  

Hard to tell without pics.