I'm not sure about "all that" techy stuff...however, I do know that the thread number is constant, as is a post number. I believe the reason the post number shows up when you select from the "last 25" section is just that...it's the last 25 posts, not threads.
Why it does whatever else you said it does, I have no clue. Maybe someone more techy will be able to explain better than I. Here are a couple of ideas/suggestions though...
1. To view the "thread #" after selecting a post from the "last 25" section, you can go up one level in the string title at the top of the page:
"Forums > Airstream Community Forums > About these forums, How-To's & more... > Thread ID # from Post ID #"
on this thread, which will put you in the forum. From there you can reselect the thread by title and the thread number will show up in your URl area. Make sense???
2. Have you tried the "view new posts" option? I have found it real helpful when I don't get a chance to check in very often...it shows all the threads with new posts since the last time you visited instead of just the last 25 posts.
3. It sounds like you are somewhat familiar with the "search" feature...you can just type in a couple of keywords and a plethora of threads relating to the topic will show up....all the way back to the beginning of time. Saves alot of note taking effort, in my opinion.
There are so many different options and ways to use the forums...after awhile, you'll feel more comfortable with the way it works. Good luck & welcome aboard!