The reason is this:
Most people who used the program, including myself, think they have a "light mix." No, they haven't developed a light mix; they aren't using
the program right. In most cases, they have developed a edgy head voice and chest voice. Not a connected voice.
You can add weight to a light mix through experimentation, but not to a edgy head voice.
So you have to ask yourself: do you truly have a light mix or just an edgy head voice? There is a difference.