advanced bone growth is assessed based on a child's rate of growth in height as well as xray findings of the left wrist called a bone age, the wrist maturity is compared to standards and the growth plates are looked at in regards to an age equivalent. Things that can lead to more mature growth plates include problems related to excess puberty hormones such as early puberty, or adrenal hormones, also obesity in itself can lead to a slightly more mature development of bones as the effect from the insulin that overweight children can make excessively in response to their size. So be sure that you understand why they did the test and the advanced bone maturity means that her growth can be affected and simply needs monitored as well as attention to the causes that may contribute.
Could be she has high androgen levels, such as high testosterone levels. Does she have other symptoms?
Has she been checked for Congenital Adrenal Hyperplasia?