Why is OnCollisionEnter2D called when it's layer's collisions are being ignored?

I'm sure this is down to my lack of understanding of the unity physics system.

I'm trying to implement a simple one-way platform for a 2d game (based on character y velocity) but have got stuck on creating a reliable way to let the character 'drop down' from a platform on pressing the down arrow.

While trying to debug and whittle down the code, I've found myself with this strange debug script:

 using UnityEngine;
 using System.Collections;
 
 public class DebugPlatform : MonoBehaviour 
 {
 
     void OnCollisionEnter2D(Collision2D other)
     {
         if(Physics2D.GetIgnoreLayerCollision (other.collider.gameObject.layer, this.gameObject.layer))
             print ("ERROR: How does this happen?!!");
         else 
             print("This is normal.");
      }
  }

When I place this on my one way platform and mess about in game, sometimes the character refuses to drop down on command, and the error message is printed.

From what I understood, GetIgnoreLayerCollision returned whether the collisions between the layers were being ignored or not.

So my questions are:

• Why is OnCollisionEnter2D being called even though the layers are supposed to be ignored?

• Is this ignore value just a request, not absolute? Maybe some collision optimization that I have to override?

• I can't imagine this is some multi-threading issue, as the error appears regularly, after I do a normal jump on the platform.

• Am I doing something completely wrong?