FAA Knowledge Test Question Bank - What is the source of the actual test questions?
Years ago (35?), I taught ground school for pilots.  The question pool was not published then (it was later published, and then withdrawn), the company would send college students to take the FAA test, with the idea that they would capture certain questions, so that they could be edited and put into study guides.  

As I recall the goal was about 5 questions per student taking a test.  Most of the students were not pilot candidates, but rather engineering and even humanities students at colleges.

Attendees would also report on small differences after they had taken the written, and the company question pool would be revised.

I cannot tell you what process is used today.