I'm going to make a guess and say that you have higher great katana skill than sword. Also, are you changing jobs for sword and great katana or are you on the same job for both? A job change could also create a STR difference, which will (slightly) affect your attack. The "DMG" rating on a weapon won't actually change your "Attack" number. The only things that will change your "Attack" number are attack+ gear(or weapons with attack+, like magian STR trials), STR gear/weapons, skill+ gear/merits and your actual skill level, and job abilities like berserk.