I'm not sure why nobody has noticed this, but it's most likely that SE couldn't be bothered to account for the weighting since that would either cause some determination (or piety) materia that you used to have no effect, or allow someone using determination and/or piety to collect fewer pieces of alexandrite. I imagine they noticed the problem and felt the best course of action would be to allow the weapon to be slightly better since it makes almost no discernible difference.