So the Unit_Defender_RACENAME_AI - with the unit name skirmisher
has the abilities
<SelectedAbilityBonusOption>ArmorProficiency2</SelectedAbilityBonusOption>
<SelectedAbilityBonusOption>Defender</SelectedAbilityBonusOption>
<SelectedAbilityBonusOption>Constitution</SelectedAbilityBonusOption>
But looking at the equipment, the unit only uses leather armor and doesn't have an armor upgrade definition. So, it will never upgrade beyond the stock armor that it is given. Hence ArmorProficiency2 is a wasted ability for the skirmisher and is costing the AI 10 extra training turns for that trait (times the number of units it trains obviously)
Might I suggest that one drops that ArmorProficiency2. I would recommend replacing it with the ability Fast. (There are a few others that do this, but I'll have to search for them again)