Jay Honeck wrote:

As both a mechanic and flight instructor, looking after six
airplanes, I can say that flat-spotting is normally a result of
improper approach speeds.


This is all right and proper, of course -- but it doesn't address the issue.

Which is: Why is one tire flat-spotted, while the other is not?

You're heavier on that brake than the other. You may be riding that
brake during takeoff or landing without knowing it.