Why? 1.006 and 1.005a are essentially the same - no added fixes or features - only "feature" of the Steam-Version is that it doesn't work on Win98.
They are only different builds.
Here I cite Lar-q, the mod from the Larian-Forums:
I can understand your confusion - the version on GOG and the version on Steam are different ports due to an accident at Larian that caused us to lose the source code of the GOG version. It's a pretty stupid thing, but sometimes things go wrong. So to make the Steam version, the port to adapt for Windows 7, had to be redone. Therefore, there's no relation between the GOG version and the Steam version, other than they're based on the same original game.
source:
http://www.larian.com/forums/ubbthreads.php?ubb=showflat&Number=456498&page=1