Part of it is that Mitchie M actually uses Miku in a reasonable range for a teenage girl/woman. A lot of Miku songs (and VOCALOID songs in general) really push the range of the vocals high. I believe part of this is because many of the producers are tenor males and compose the vocals for a tenor male, then raise it an octave for Miku, resulting the range being above soprano. The other part of it is probably simply because unlike a human singer, a VOCALOID can just keep singing higher and higher, so there's no feedback telling the composer its too high for the vocalist.
The other part of Mitchie M's tuning is Melodyne.