Mr.Green Hat pulls the string so that the pitch of the open string is correct.

  Mr.Red Hat got on the 1st fret, believing that the position of fret was correct, so he could make a perfect semitone.

  Since Mr.Red Hat got on the string, Mr.Green Hat had to pull with more force than ever.

  A big thing happened here.

  Mr.Green Hat increased the tension of the string and pulled it, so the pitch of the semitone became a little sharp.

  The phenomenon that the pitch sharpens when you hold down a string like this occurs in a normal guitar.　

   MTS is the way to get rid of this.