The versions posted here are development builds, i.e. always the latest changes the developers have made to the code. On the sourceforge (not soulforge, nice unintended pun btw
) page, only so called stable releases are published. This is pretty much standard in open source projects.
The stable builds are guaranteed to be quite playable, whereas the development builds can and most likely will contain some bugs. Their reason is actually for people to test them and find those bugs - so the devs can fix them. Normally to get them you'd have to check out the source code and compile the game for yourself (i.e. have at least basic knowledge of coding), but Destructavator (and odie before) are so nice to compile and upload them, so more people (who can not or do not want to compile them themselves) can test it.
If you compare it to commercial games, playing the development builds is like partaking on an open beta. You'll have to endure a bug or two here and there. And it would be nice if you report them in the forum, if you find any. On the plus side, you'll get a lot of new features that wasn't in the last stable release.
Especially for ufoAI, where the last stable release was ages ago, much much has changed.