just saving them all the time.

That is to say, I think I may have figured out the answer to this. (The operative word being may).

ABC News:

But supervised release a form of probation is available only to people who have served prison time.

It seems that there are different forms of probation. Sandy Berger must have been sentenced to a form different from supervised release.

Some lawyer would have to confirm this. But in view of the fact that no lawyers came forth to challenge Judge Walton, and fling Sandy Berger in his face, I suspect this is correct.