If you turn your fill up above your main light, it's no longer fill light.
Your main/key light is your primary light that is providing most of the light on the face. Your fill is used to fill in the shadow areas that the main light creates.
Here are two examples:
No fill, with a single light to camera right:
See how the shadow on my right cheek is dark?
In this one, the fill is two stops under the main light:
The nose shadow is 'filled in' by the fill light.
In the second one, if I had put the fill two stops higher than the key light, it would no longer be the fill, and would completely overpower the main light.