Listed accuracy is not added, it's the final multiplier used on the result of the formula that calculates melee accuracy from skills. Which tends to depend on the weapon.
E.g. a machete with an accuracy multiplier of 80% and 0.5 melee + 0.5 flat accuracy in the hands of a 50-skill agent gets you (0.5*50+0.5*100)*0.8 = 60% hit 'chance'. That's before that invisible melee dodge comes and fucks up the result, of course.