In breathing mindfulness ,why only long and short breaths are taken into consideration?
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.