Fairolniza, my experience with multiple crystal forms growing from the same drop (I had 4 that I was able to identify, there could have been more) was that there were similarities in the way protein molecules packed into "fibers" that were subsequently arranged into distinct crystal forms. You may want to consider looking at the protein-protein interfaces in your two crystal forms using PISA server and see if there are similarities. IMHO, any theory explaining why microgravity would favor one of the two forms would be pure speculation (with additional caveat that there is no way you can make two experiments completely identical in all other respects anyway), but what's wrong with that? :)
Cheers, Ed. -- "I'd jump in myself, if I weren't so good at whistling." Julian, King of Lemurs