i wonder why is the damage of friede's scythe low?

So i have +5 friede's scythe and it has 389 physical damage output. And i also have a +10 sharp uchi and it has 391 physical damage output. Both weapons deals slash damage but when I'm fighting demon prince, scythe deals 287 and uchi deals 357 damage. I wonder why there is such a big gap between damage outputs while stats say there is only 2 points of difference. Am i missing something?