I think there must be a percentage of mission types. I find that my normal "spread" of missions is: 2-3 metal/cloth missions, and the rest comp missions. the metal/cloth missions seem t randomly choose one or the other, and not sure about the grade of mission (i.e. moderate, abundant, rich, etc). those could be percentage based all the missions for that tier...though I never get rich metal/fabric.
I KNOW this to be false. I have actually had a grade 6 list have no gift missions. Ironically, it happened when I was doing a lot of gift missions for an alt. And there have been plenty of times when it is ALL gift missions (I've experienced it and a LOT players have come to these forums to complain about it).
Also grade 6 materials missions never go above abundant. grade 6 companion gift missions are ALL rich grade 5.