The Actual Landing Distance as you know is calculated with a lot of factors standardised. To mention some of these .
- The a/c is at 50ft above runway threshold
- the approach speed is at Vr + 5
- the pilot applies max braking
- antiskid is operating
- the ground spoilers are operating
- thrust reversers are not taken into account
and some unclear factors as:
- the wear of the tires
- the wear of the braking system
- actual runway surface friction
So no big deal to understand that the actual landing distance is your final point when considering a suitable runway.
When in flight you experience an emergency (which means you have to land at nearest suitable airport) you have to check the actual landing distance and need not factor it with the above values. But if you are going to land at the exact value you shouldn't forget the above assumptions.
The point is you have little room to play when you are planning to land to a runway that is exactly or close to your actual landing distance reqired.
Hence, for normal operations you are factoring the actual landing distance required with the above values to cover for factors like,
- slight airspeed variations
- slight tail winds
- long flare
- normal braking (passanger comfort and money)
- not new tires
- etc.
Hope it helped.
Guclu