OK, I think I know where I was having difficulty with the armor. Bear with me please...
Damage Divisor for a Large Beast Mount = 1, plus the DD for "Clothing, Fatigues/Civilian/Non-Armored" of 1 = 2 total. This is the same divisor as the Periphery Armor Kit worn by the Tariq troopers. Clothing, Light (e.g. Summer Wear/None) is a 0.5, but I wouldn't consider the tribesman's attire to be summer wear despite being worn in a desert.
So...
15 / 2 [DD] = 7.5
21 [Troops] / 7.5 = 2.8 = 3 [Rounded Normally]