Sharpness on the axe

I was playing on the server when suddenly I find a diamond axe with a sharpness of 5 in a new dungeon. Despite the damage, I go back and suddenly notice that it does not differ in damage from the usual iron and then I bought a book for sharpness 4 and tried to enchant an iron axe for sharpness 4 when suddenly I see that the damage is not added. Please fix this