Here is a list of blocks which appear to be pre-set to land in 'side-post' regardless.
Course Overview (default-region: = region-content)
Private Files (default-region = side-post)
Online Users (default-region = side-post)
Latest Badges (default-region = side-post)
Calendar Monthly View (3 months) (default-region = side-post)
Calendar Events Key (default-region = side-post)
Upcoming Events (default-region = side-post)
Recent Activity (default-region = side-post)
Search Forums (default-region = side-post)
I found all this out in a new install, when visiting different pages you find the blocks in odd places. So with Editing on you start to re-configure these block settings, the options in the drop-down list shows 'side-post' yet all the Lang files use 'Left' or 'Right'. So this suggests to me there is some CORE coding going on that will (as you have found) contradict with some themes.
For example set a new block region in a theme and make it span12, which only works well with HTML blocks that you can add HTML to which can transform into Marketing areas as described in the LOOK & LEARN tutorial.
So if you set that new block region to 'default-region' all these blocks get added to it. Which can be frustrating as I am finding in my new GoLogo theme which I am currently working on. Although I have managed to create quite a nice layout for the Events Key and the Monthly View.
Hope this helps?
Cheers
Mary