Why can’t I see who all the friend shared a post with

In composing and editing the question, I might have come up with the answer myself:

• "Public" doesn't broadcast my post as widely as "Extended Circles" does. Yes, it's available but only to those who actively come looking.
• "Extended Circles" broadcasts my post not only to my circles but also to theirs. This gives me more visibility. But strangers can't see the post even if they come looking, unless they happen to be only semi-strangers because they are in the "extended circles".

So "Public" provides availability while "Extended circles" provides distribution. These are separate traits.

Frankly though, why would I write something that semi-strangers may read, but total strangers may not? That is what the "Extended circles" does, and it doesn't make much sense to me. So in essence, I would always use both in combination.

You can see the whole history of the Post via the Ripple of the Post: https://plus.google.com/ripples/details?activityid=CzWHefUKR1T

You can get to the Ripple for popular, shared Posts by clicking the Post's context menu (the down arrow on the right) and selecting View Ripples.

I think in the post you described, Peter Norvig is the author of the first post. He may have cut & paste the "patient zero" posting instead of sharing the original, if "patient zero" was the first?