Since no one is answering this I’ll try to at least point you to a direct answer, although I never use the function myself.
From the FAQ:
In practice, thread susbsription can be spotty. For reasons unknown, I have noticed that sometimes either the subscription doesn’t “take”, or else that it will mysteriously stop in mid-thread. Some of this could be a combination of server load/mail server load issues, but it would require an answer from jdavis to get the Straight Dope on that point.
Believe me, I FAQ frequently, and I don’t think it addresses the issue. To vB, Subscribing apparently is a different function from receiving an email notification.
I have no trouble getting email notifications, and that option defaults to ON in my profile. But I use the “user cp->subscribed threads” page view to see what is happening with topics I am actively posting to, and I have found that a mere new posting in a thread does not always enter the thread in that subscribed list. I might be considered subscribed for the purposes of emailing, but not for the CP list.
After starting this thread, I checked the list, and I had been subscribed to it. My guess is the thread start function is more solid than the thread post function. Maybe that is what you have noticed, Anthracite.
I can be sure to get subscribed by clicking on the button on the page after I post; it’s just an extra step that I wish wasn’t necessary.
However, this does not address your question of why the apparent action may be different between the two. Searching on their forums over there (which anyone can do, and I highly recommend it for people that want to know more) reveals many and sundry issues with odd notifications and subscription behaviour.
Sadly, as I said, I never use it so don’t have huge experience. And it was disabled on my own Board until a few months ago. But maybe you can find an answer on the vBulletin site, if you can search on better and more exact terms than I seem to be using.
A long time ago, I posted a question on the vB board about this phenomena, but got no answers, and I didn’t pursue it. I thought the answer would come to light over time.
Just now, I did a test on the Door County Message Board where I have full admin control. I also have a few non-admin names for test purposes. That board runs vB 2.2.1, a slightly older version than SDMB.
First, I logged in as admin and deleted all subscriptions to both threads and forums. Next I started a new thread. Back at CP, no thread showed as subscribed (and I did a refresh each time I revisted a page during this test). Next I posted a reply to the same message. Back at CP, nothing showed.
I logged off, then on again under the same name. In CP, still no subscriptions. So I logged in under a non-admin name, posted a reply to the same thread, and the thread showed up in that user’s subscription list!!
Went back to the admin user’s CP, still nothing. This doesn’t make any sense. The virtues of admin wouldn’t apply in SDMB, since I have no privileges greater than the lowest user.
Did the clearing out of all subs have an effect (because I skipped that step for the second user name)? Dunno, but it’s probably not worth spending any more time on it unless someone has a clue of what to look for.