Please excuse me if this question seems trivial to art and painting experts but recently I became aware that besides the Louvre version of Mona Lisa, there is another version called Mona Lisa Prado. I know it's already known that Mona Lisa Prado is painted relatively at the same time when Louvre's Mona Lisa painted in Da Vinci's studio. Also, I'm aware that despite several similarities between the Louvre's Mona Lisa and the Prado version, there are significant differences between these two paintings. The most important information that I found is that based on experts' opinion, the Prado version is also painted in Leonardo Da Vinci's school probably by Salai or Francesco Melzi or other Spanish Da Vinci's pupils.
My question is that if the Prado version is painted at the same time as the Louvre's version and it comes from the Da Vinci's studio, how do we know that the Louvre's version is painted by Da Vinci himself but the Prado version is probably painted by one of his pupils? In all articles about the Prado version, the main hypothesis is that the Louvre's version is the masterpiece of Da Vinci but the Prado version is just a copy by Salai or Melzi, but nobody describes why art experts think that is the case. I really appreciate any suggestion or hint here.