This range of weights is so wide, because kids do tend to grow at varying rates. It is therefore possible for five 12 year old girls to all be of a different height.
In addition, the basic structure of their bones will also influence their actual weight. A girl with a very heavy bone set, for instance, will automatically be heavier than a girl with slender bones.
It becomes obvious how difficult it is to determine how much a girl should weigh at the age of 12, or any other age, for that matter.
The main thing to remember is that there is no such thing as a perfect weight. Not everyone can be perfectly skinny. At this age, being a little chubby may be as normal and healthy as being a little on the skinny side.
As long as the girl is fit and healthy, with plenty of exercise and a healthy, balanced diet, all is well. The 'diet' in this, by the way, refers to the general food intake.
A girl of this age should under no circumstances consider going onto a diet in order to lose weight, unless instructed to do so by medical professionals due to being dangerously overweight.
If extremely high or low weight do constitute a reason for concern, it is best to consult a doctor, who will be able to determine whether there is a problem, and if so, how best to proceed in order to correct it safely.