This is a known bug. When you fight in a battle involving nearby lords joining your side, the autocalc does not take their strength into account when measuring the battle advantage. This results in the calculator treating your army of 1000 as a mere 90 (your party's size), but your allied lords still take the damage.
link to bug report: http://bugs.taleworlds.com/edit_bug.aspx?id=1821
link to bug report: http://bugs.taleworlds.com/edit_bug.aspx?id=1821

