How does combo orbs work

The 220% is part of another formula that results at a 1.9 multiplier

 

The way it works with combo attack is that the "weapon attack +220%" just means 120%.

The skill effects description is very misleading the formula is
(Combo attack % - 100) + floor ((orbs-1)*(combo attack skill level/6) [0-5 orbs]

So if your combo attack is level 30 and you possess 5 orbs it is
(220 - 100)% + floor((5-1)*(30/6)) = 140% damage boost so 1.4x multiplier.

With advanced combo attack level 30 and 10 orbs it is
(Combo attack% + aca% - 100%) + 20% (5 orbs) + (orbs - 5) * 4% <- [6-10 orbs]

(220+30-100) + 20% + (10-5) * 4 = 190% so a 1.9x multiplier.