I may have found an experimaental bias. Confirming....
Edit:
In subsequent tests I am finding another ‼Science‼ data point which invalidates this experiment:
It appears that if two dwarves are placed during the same tick, the one placed first gets to swing first. When wrestling, this had little consequence. When armed with platinum war hammers vs. cloth, the first strike advantage is huge. Running symmetrical full scale tests in both directions I am not seeing the protective difference in clothing anymore and they appear to be equivalent in protection. The fully armored tests take longer and I will re-run those symmetrically to see if there is a difference, as the 5% seen in the first test is easily explainable by the first strike bias. My apologies for the misleading ‼Science‼, don't forgo the turkeys just yet.
Edit2:
I re-ran the experiments, mirroring each in two experiments so that each experiment had the same bias but in the opposite direction.
Pig tail and leather clothing appear to be equivelent in protection against war hammers. Since leather is lighter, it's probably the better material. I did not do silk/yarn.
Adding clothing under steel armor seems to give a slight advantage. With the bias in favor of no clothes, we get about 52% wins. With it in favor of clothes we get 55% wins. It's a tiny advantage though and may be statistical noise. The clothes chosen were leather dress, robe, cloak, hood, mittens, trousers and pig tail socks with no multiple items.
Overall the weird anomolies are gone from my first experiment. Leather and fiber clothes appear equivalent, and add very slight protection when used in conjunction with armor and probably aren't really significant at the steel level (they may be at the copper level).
I couldn't find any solid information on whether leather and pig-tail cloth were equivalent in protection, so I deceded to run some tests. The results were rather interesting.
First off, just to make sure everything is configured correctly, I ran 1000 1-on-1 battled between 2 buck-naked dwarves, the first of which had a steel battle axe, and the second was unarmed. Needless to say, there was a 100% casualty rate amongst the unarmed dwarves. Good, everything works.
Lesson 1: Don't run naked and unarmed into battle when your opponent has a battle axe. Even if the opponent is naked too, you are going to have a bad day.
So, now I try wrestling. 2 dwarves, no skills, no weapons, both in clothes only: Robe, Dress, Cloak, Hood, Mittens, Trousers, Shoes. The first is in all Pig leather and the second in all pig-tail cloth. Unsurprisingly, the results are almost 50-50. No significant difference when wrestling.
Third test: I give each a copper battle axe while fully clothed. This test is much quicker. Interestingly, 75% of the wins go to the pig tail cloth! Leather looks inferior in this case. This leads me to do some deeper testing.
Fourth test: I swap out the battle axe with a platinum war hammer. I figure maybe the numbers will flip when bashing. Nope. This time, 90% of the wins go to the pig tail cloth! Amazing! I think my leather industry will be much less busy from now on.
Lesson 2: Leather sucks. Pig tail cloth is much better.
Fifth test: I wonder how clothing+Armor compares with Armor alone. I give each unskilled dwarf a steel battle axe, mail shirt, breastplate, helm, greaves, gauntlets and high boots. The second dwarf I add a pig tail robe, dress, cloak, hood, mittens, trousers and socks. 55% of the wins go to the *unclothed* dwarf.
Okay, so it's probably the lack of Armor User skill which is letting the unladen dwarf gain a 5% edge I think. So....
Sixth test: Same as the fifth but give each dwarf grand master armor user skill. Anomoulously, the unclothed dwarf still wins, this time with 56% of the wins, so skill makes no difference at all!
Lesson 3: Not only do clothes not help a fully armored dwarf, they appear to hinder that dwarf slightly even when highly skilled vs. that same dwarf having no clothes.
More testing is needed to determine what my civilian uniform should be, but I think my military will now be clad in metal only. No socks for them, which means less laundry to do!