Yanny or Laurel? Why This Happen?

First of all watch this video:

While some people claim to hear the word "Laurel" in the clip, others say they hear "Yanny."
The Clip has been shared thousands of times, and people around the world – including celebrities Chrissy Teigen, Mindy Kaling and, of course, Yanni – have weighed in on what they hear. 
The internet erupted in disagreement on Tuesday over an audio clip in which the name being said depends on the listener. Some hear “Laurel.” Others hear “Yanny.”
The clip and original “Yanny or Laurel” poll were posted on Instagram, Reddit and other sites by high school students who said that it had been recorded from a vocabulary website playing through the speakers on a computer.

One detail may frustrate some and vindicate others: The original clip came from the vocabulary.com page for “laurel,” the word for a wreath worn on the head, “usually a symbol of victory.”
One way to understand the dynamics at work is to look at a type of chart called a spectrogram — a way to visualize how the strength of different sound frequencies varies over time. The spectrograms above show that the word “laurel” is strongest in lower frequencies, while a simulated version of the word “yanny” is stronger in higher frequencies. The audio clip shows a mixture of both.
By using the slider to manipulate which frequencies are emphasized, it makes one word or the other more prominent.
Why This Happens??