In the BS 7671 current capacity tables, armoured cable has a higher rating than unarmoured cable.
Given current capacity is a heat balance - energy loss within the cable making heat, which is then dissipated such that the insulation temperature is not exceeded. Unarmoured and armoured cables have the same I2R loss (heat gain). However armoured cables also have additional losses generated within the armour due to circulating currents. This extra heat should result in a reduced current rating.
Why is this not the case in BS 7671.