Heavy has a different twist rate better suited for heavier pellets and heavier slugs (as opposed to lighter weight Hybrid Slugs).
I wonder what they mean by "Heavier" pellets?
And I just answered my own question. Same thread down in the comments from FX:
So the Standard Superior STX is for pellets, hybrids, and possibly some of the lighter weight slugs from other manufacturers. The Heavy Superior STX is for heavier slugs and even some heavier pellets (think Monster Re-designs in .22 / 50gr .30 Cal pellets / etc)