Originally Posted by Zandel

The reason the 5* girls takle less damage than the 6* is most likely because of equipment buffing the 5* girls to 1-2k more Defence than the 6* girls. 6* with full Equip slots take no damage at all in a -70% damage team.

I actually noticed the same thing on my own teams (including the debuff squad), where most 6* girls also have full equipment and are fully amped up. It's kinda like there is some odd modifier at work that caps the effect of DEF for 6* girls stronger than it does for 5* girls.