You said you used him at 3.60, that’s why. His final stat attack at 4.70 is 541, kinda low, so imagine using it with even lower stats. (Spoiler: he kinda sucks even at 4.70, and he received two buffs through the years if I’m not wrong).
And NO, his special is not working different from Li Xiu or Hu Tao, they’re working same identical way, if you’re familiar with how Damage Calculation work and its formula you should know it. Otherwise I advise you to check that specific thread, it’s very helpful.