For Anti-skid the BSCU uses ADIRU speed as a "reference speed" for releasing brake pressure to a skidding wheel. No ADIRU = No reference speed, so only the individual wheel speeds are available for comparison with each other. A conservative deceleration target is used, which I believe has been increased for later aircraft. Hence the large factor.
More gen from your training staff if they look at the Airbus October Standards Meeting Report.