Maybe some years ago they imported them and now you see them on the legion.
This is a relevant question. Decay and temporary assets should be reflected as an expense. You can have the good or men (trade and manpower) but what represents the replacement when they die or retire?
For manpower that could be monthly attrition, for units that require goods, that could be simulated by additional monthly attrition for that unit that you do not have a surplus, that cannot be replaced by manpower until you get the surplus back again.