We've already found that the system factors in your previous missions to influence the new missions available. The more you run metal missions the more rare they will become.
Do your tests again but only run fabric missions for a week and the see how rare your metal missions are then.
I can prove this right now. There was a point when all I wanted was companion gifts (I was trying to raise affection on an alt). I ran grade 5 and 6 gift missions with all my companions over and over and over again, and then there was a point where all the grade 6 missions were fabrics and metals (both abundant and three moderate). I took full advantage of the situation of course
From that point on, I have run a healthy mix of missions and never fail to get the abundant metals mission at least once a night. The way I figure it is if I do not use the materials or gifts someone else will (guildie, friend, or off the GTM).