The versioning has been explained many times on the Scribus mailing list and the background is something like that one branch that was supposed to become the text stable unfortunately went into a dead end due to (I think) a new version of Qt. So the 1.3.3-series had to be "promoted" into stable becuase it would take to long to wait for 1.4.
So, from now on (hopefully), 1.<even number> will be stable versions and 1.<odd number> will be development versions.