Malbim: Hashem conducts those who guard His Torah with two Midos - Mishpat and Tzedakah. Mishpat is according to their merit and virtue. The extra good that Hashem does with Tzadikim is based on supreme Tzedakah. Orach is a path for the Rabim. Nesiv is for individuals. Mishpat goes in individual paths - each person is rewarded based on the path he went in, and his deeds. Tzedakah goes on the path for the Rabim.