The mitigation factor corresponds to your team, taking first the shield depth scutum Def: 25, if you do not have a shield that of your weapon magic sword Def: 35, in this case your weapon has more Def than the shield for that reason it goes down the mitigation when you use shield.
Another thing to keep in mind is that the sword has additional Def to the shield, for example magic sword (Atk:48, Def:35 +3). + depth scutum (Def:25, magic level +2). Your total Def for the mitigation calculation is 25+3 =Def 28 the same mitigation would be given by a bonelord shield (Def:28).