HEALT -= Damage; // is the same as HEALT = HEALT - Damage;
HEALT --;        // is the same as HEALT = HEALT -1;

You first subtract the actual damgage from your health then you subtract additional 1.

As @zyzyx said, you're actually applying Damage+1. You could make your code more clear by just subtracting Damage from HEALT, and improve the messaging by using a single SendMessageUpwards (SendMessage functions are a bit slow):

function FallDamage(Damage :float ){
  HEALT -= Damage;
  Debug.Log("I'm falling");
}

// send message:
  ...
  function FallingDamageAlert (fallDistance : float) {
    ...
    if (fallDistance >= 15){   // if fallDistance >= 15...
      var damage: float = 5;   //  assume damage 5... 
      if (fallDistance >= 20){ //  but if it's >= 20...
        damage = 25;           //  change damage to 25
      }                        // apply the specified damage
      gameObject.SendMessageUpwards ("FallDamage", damage, SendMessageOptions.DontRequireReceiver);
      Debug.Log ("Ouch! Fell " + fallDistance + " units!");  
    }
  }