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'"?