400 meter margin may sound like a lot, but is it? Those ALD performance figures are not real, they were never demonstrated properly. The aircraft manufacturer is allowed to combine the best segments from different flights: Best flare, best touch down, best braking - and add each up a combined ALD. The PERFECT (unachievable) landing, made by a test pilot, with no regard for pax comfort and probably with little fear of making a hard landing and damage to the aircraft. Probably they had perfect tires and brakes installed as well.
You have it the wrong way around. The certified landing distance in the AFM is constructed that way, but tyres and brakes are degraded to service life, the result is then multiplied by 1.67. The advisory landing distance is "real world" with allowance for line pilot operations and the inclusion of reverse thrust and in some cases a further 15% is added.
The International Federation of Airline Pilots has the best description of the difference between Certified and Advisory here:
http://www.ifalpa.org/downloads/Leve...20aircraft.pdf