Why does the Mona Lisa have that hairstyle?

I often look at catalogs of classical paintings as material for collages.
What I notice is that women usually have their hair tied up, and not many of them wear it down.
Mona Lisa's hairstyle is actually quite unusual. In fact, in the 16th century, only girls and prostitutes wore their hair down.
It is strange that the wife of a wealthy merchant, who is considered a likely model, has that hairstyle.

As expected, a recent analysis of the Mona Lisa by the National Research Council Canada using 3D technology suggests that she originally had her hair tied up and may have been redrawn with her hair down at a later date by Da Vinci. 
On the other hand, there is a hypothesis that the Mona Lisa is a portrait of da Vinci himself, based on the skeletal structure and other features.

It is also sometimes suggested that it is a projection of the image of his mother, who left him as a young child to marry into another family. 
Then, not surprising that mother and son should resemble each other.

He kept this painting with him until his death, and I myself imagine that her hair suggests an idealized, yet puzzling, mother as a girl and a prostitute.