In breathing mindfulness ,why only long and short breaths are taken into consideration? I have just started practicing breathing mindfulness.

At several places following is said for the breathing mindfulness:


  While breathing in long, he knows “I am breathing in long”, or, while
  breathing out long, he knows “I am breathing out long”; or, while
  breathing in short, he knows “I am breathing in short”, or, while
  breathing out short, he knows “I am breathing out short”.


However most of the time I am breathing normally. I am neither taking long breath nor short breath.

My question is: Why normal breathing situation is ignored during breathing mindfulness? In other words ,why don't we say "While breathing in normal , he knows 'I am breathing in normal', or , while breathing out normal, he knows ' I am breathing out normal'"?
I can guarantee that you can take normal breathing. And you should take normal breath. Without trying to make it long or short.


  While breathing in long, he knows “I am breathing in long”, or, while breathing out long, he knows “I am breathing out long”; or, while breathing in short, he knows “I am breathing in short”, or, while breathing out short, he knows “I am breathing out short”.


As you state, Normal breathing is not ignored but it just a way to teach meditators.

The goal of this sentence is you should know every single moment of breath.