Why don't people breathe equally out of both nostrils?

Before going into the details , let us experiment this first so that you may believe me from the start.

If you get closer to a mirror and breathe out some air , fog will pop up on the mirror. Marks of water vapor will pool up on the surface of mirror facing you. But deeply we notice it , one mark will be larger than the other. So this implies that we breathe mostly out of one nostril at a time.

So why do we breathe mostly out of one nostril ?

At any instant of time , 75% of breathing is done through one of the nostrils and rest 25% through the other, said Dr. Michael Benninger, who works as a head-and-neck doctor at the Cleveland Clinic. The former nostril would be momentarily called as the dominant nostril and it switches throughout the day. This is known as  Nasal cycle.  

During this nasal cycle, one nostril becomes congested  and hence contributes less to the airflow while the other gets decongested. So consequently we breathe more through the former nostril. After sometimes the process switches between the two nostrils. Now we are supposed to know how much it takes to switch the cycle. According to a small 2016 study published in the journal PLOS ONE,  the above cycle switches about every 2 hours on average.  Interestingly , the study also observed that the right handed people tended to spend more time favoring their left nostril.

Benninger says that "No one is sure why the nasal cycle occurs". But there is a popular theory regarding this which says " It is  speculated that it has to do with allowing moisture to build up on one side so that it doesn't get too dry."

He also explains that most people aren't aware of the nasal cycle. However, it may become significantly noticeable during sleep. For example if a person lies on their right side, gravity will cause that lower nostril — the right nostril — to become more congested. If the cycle has designated the right nostril to be naturally more congested at the time, there's no appreciable effect. But if the nasal cycle has made the left nostril more congested and the right nostril is congested because of side sleeping, breathing may be difficult, and the person may wake up.

People will usually notice it if one of the nostrils is consistently blocked. Sometimes this may happen due to a person has a  deviated septum, a condition when the wall between the  two nostrils is displaced which pushes it to one side. Some people also grow soft, painless growths — called polyps — in their nose, which may produce the same effect. 

When the person suffers from Common cold, then in general both the nostrils are congested , so the person feels trouble in breathing no matter where he is in the nasal cycle.

However , there exist ways to decongest both the nostrils at once which can temporarily lead  to breathe more equally out of both nostrils ,  until the nasal cycle resumes. Medications such as nasal decongestant sprays, as well as exercise, can have the same effect.