I think you’re looking at the Level 1 Special Skill on the Costume.
The Level 8 Special Skill for Li Xiu’s Costume is 180% damage, vs. 150% damage for her regularly.
Her stats are also built a little differently on each, she has a little higher Attack on her Costume, so the Special Skill amount is effectively higher.
180% x 660 = 1188
vs.
150% x 616 = 924
She also gets a Costume Bonus in either Costumed or regular form, so even if you don’t use the Costume, it effectively upgrades her regular form.
I like Li Xiu’s Costume on Offense in some cases, because she hits harder, and is a Rogue, which gives her the ability to dodge Special Skills.
I have my base Li Xiu at +18, here are her stats in each of mine:
That makes the 150% vs. 180% a more pronounced difference, since her Attack stat is considerably higher in Costume.
150% x 676 = 1014
vs.
180% x 739 = 1330
I like using her in Costume as a Rogue in my mono yellow “nope squad,” which is themed around avoiding or overriding damage with dodging, blinding, mana cut, and cleansing — Inari, Joon, Jackal, Costumed Li Xiu, and Vivica.
The 10% mana cut works well there, because it gives me time to stall enemies slightly so I can set off some intentionally to trigger Inari’s dodges.